//! Panels are [`Ui`] regions taking up e.g. the left side of a [`Ui`] or screen.
//!
//! Panels can either be a child of a [`Ui`] (taking up a portion of the parent)
//! or be top-level (taking up a portion of the whole screen).
//!
//! Together with [`crate::Window`] and [`crate::Area`]:s, top-level panels are
//! the only places where you can put you widgets.
//!
//! The order in which you add panels matter!
//! The first panel you add will always be the outermost, and the last you add will always be the innermost.
//!
//! You must never open one top-level panel from within another panel. Add one panel, then the next.
//!
//! ⚠ Always add any [`CentralPanel`] last.
//!
//! Add your [`crate::Window`]:s after any top-level panels.

use crate::{
    lerp, vec2, Align, Context, CursorIcon, Frame, Id, InnerResponse, LayerId, Layout, NumExt,
    Rangef, Rect, Sense, Stroke, Ui, UiBuilder, UiKind, UiStackInfo, Vec2,
};

fn animate_expansion(ctx: &Context, id: Id, is_expanded: bool) -> f32 {
    ctx.animate_bool_responsive(id, is_expanded)
}

/// State regarding panels.
#[derive(Clone, Copy, Debug)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
pub struct PanelState {
    pub rect: Rect,
}

impl PanelState {
    pub fn load(ctx: &Context, bar_id: Id) -> Option<Self> {
        ctx.data_mut(|d| d.get_persisted(bar_id))
    }

    /// The size of the panel (from previous frame).
    pub fn size(&self) -> Vec2 {
        self.rect.size()
    }

    fn store(self, ctx: &Context, bar_id: Id) {
        ctx.data_mut(|d| d.insert_persisted(bar_id, self));
    }
}

// ----------------------------------------------------------------------------

/// [`Left`](Side::Left) or [`Right`](Side::Right)
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Side {
    Left,
    Right,
}

impl Side {
    fn opposite(self) -> Self {
        match self {
            Self::Left => Self::Right,
            Self::Right => Self::Left,
        }
    }

    fn set_rect_width(self, rect: &mut Rect, width: f32) {
        match self {
            Self::Left => rect.max.x = rect.min.x + width,
            Self::Right => rect.min.x = rect.max.x - width,
        }
    }

    fn side_x(self, rect: Rect) -> f32 {
        match self {
            Self::Left => rect.left(),
            Self::Right => rect.right(),
        }
    }
}

/// A panel that covers the entire left or right side of a [`Ui`] or screen.
///
/// The order in which you add panels matter!
/// The first panel you add will always be the outermost, and the last you add will always be the innermost.
///
/// ⚠ Always add any [`CentralPanel`] last.
///
/// See the [module level docs](crate::containers::panel) for more details.
///
/// ```
/// # egui::__run_test_ctx(|ctx| {
/// egui::SidePanel::left("my_left_panel").show(ctx, |ui| {
///    ui.label("Hello World!");
/// });
/// # });
/// ```
///
/// See also [`TopBottomPanel`].
#[must_use = "You should call .show()"]
pub struct SidePanel {
    side: Side,
    id: Id,
    frame: Option<Frame>,
    resizable: bool,
    show_separator_line: bool,
    default_width: f32,
    width_range: Rangef,
}

impl SidePanel {
    /// The id should be globally unique, e.g. `Id::new("my_left_panel")`.
    pub fn left(id: impl Into<Id>) -> Self {
        Self::new(Side::Left, id)
    }

    /// The id should be globally unique, e.g. `Id::new("my_right_panel")`.
    pub fn right(id: impl Into<Id>) -> Self {
        Self::new(Side::Right, id)
    }

    /// The id should be globally unique, e.g. `Id::new("my_panel")`.
    pub fn new(side: Side, id: impl Into<Id>) -> Self {
        Self {
            side,
            id: id.into(),
            frame: None,
            resizable: true,
            show_separator_line: true,
            default_width: 200.0,
            width_range: Rangef::new(96.0, f32::INFINITY),
        }
    }

    /// Can panel be resized by dragging the edge of it?
    ///
    /// Default is `true`.
    ///
    /// If you want your panel to be resizable you also need a widget in it that
    /// takes up more space as you resize it, such as:
    /// * Wrapping text ([`Ui::horizontal_wrapped`]).
    /// * A [`crate::ScrollArea`].
    /// * A [`crate::Separator`].
    /// * A [`crate::TextEdit`].
    /// * …
    #[inline]
    pub fn resizable(mut self, resizable: bool) -> Self {
        self.resizable = resizable;
        self
    }

    /// Show a separator line, even when not interacting with it?
    ///
    /// Default: `true`.
    #[inline]
    pub fn show_separator_line(mut self, show_separator_line: bool) -> Self {
        self.show_separator_line = show_separator_line;
        self
    }

    /// The initial wrapping width of the [`SidePanel`], including margins.
    #[inline]
    pub fn default_width(mut self, default_width: f32) -> Self {
        self.default_width = default_width;
        self.width_range = Rangef::new(
            self.width_range.min.at_most(default_width),
            self.width_range.max.at_least(default_width),
        );
        self
    }

    /// Minimum width of the panel, including margins.
    #[inline]
    pub fn min_width(mut self, min_width: f32) -> Self {
        self.width_range = Rangef::new(min_width, self.width_range.max.at_least(min_width));
        self
    }

    /// Maximum width of the panel, including margins.
    #[inline]
    pub fn max_width(mut self, max_width: f32) -> Self {
        self.width_range = Rangef::new(self.width_range.min.at_most(max_width), max_width);
        self
    }

    /// The allowable width range for the panel, including margins.
    #[inline]
    pub fn width_range(mut self, width_range: impl Into<Rangef>) -> Self {
        let width_range = width_range.into();
        self.default_width = clamp_to_range(self.default_width, width_range);
        self.width_range = width_range;
        self
    }

    /// Enforce this exact width, including margins.
    #[inline]
    pub fn exact_width(mut self, width: f32) -> Self {
        self.default_width = width;
        self.width_range = Rangef::point(width);
        self
    }

    /// Change the background color, margins, etc.
    #[inline]
    pub fn frame(mut self, frame: Frame) -> Self {
        self.frame = Some(frame);
        self
    }
}

impl SidePanel {
    /// Show the panel inside a [`Ui`].
    pub fn show_inside<R>(
        self,
        ui: &mut Ui,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.show_inside_dyn(ui, Box::new(add_contents))
    }

    /// Show the panel inside a [`Ui`].
    fn show_inside_dyn<'c, R>(
        self,
        ui: &mut Ui,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let Self {
            side,
            id,
            frame,
            resizable,
            show_separator_line,
            default_width,
            width_range,
        } = self;

        let available_rect = ui.available_rect_before_wrap();
        let mut panel_rect = available_rect;
        let mut width = default_width;
        {
            if let Some(state) = PanelState::load(ui.ctx(), id) {
                width = state.rect.width();
            }
            width = clamp_to_range(width, width_range).at_most(available_rect.width());
            side.set_rect_width(&mut panel_rect, width);
            ui.ctx().check_for_id_clash(id, panel_rect, "SidePanel");
        }

        let resize_id = id.with("__resize");
        let mut resize_hover = false;
        let mut is_resizing = false;
        if resizable {
            // First we read the resize interaction results, to avoid frame latency in the resize:
            if let Some(resize_response) = ui.ctx().read_response(resize_id) {
                resize_hover = resize_response.hovered();
                is_resizing = resize_response.dragged();

                if is_resizing {
                    if let Some(pointer) = resize_response.interact_pointer_pos() {
                        width = (pointer.x - side.side_x(panel_rect)).abs();
                        width = clamp_to_range(width, width_range).at_most(available_rect.width());
                        side.set_rect_width(&mut panel_rect, width);
                    }
                }
            }
        }

        let mut panel_ui = ui.new_child(
            UiBuilder::new()
                .id_salt(id)
                .ui_stack_info(UiStackInfo::new(match side {
                    Side::Left => UiKind::LeftPanel,
                    Side::Right => UiKind::RightPanel,
                }))
                .max_rect(panel_rect)
                .layout(Layout::top_down(Align::Min)),
        );
        panel_ui.expand_to_include_rect(panel_rect);
        panel_ui.set_clip_rect(panel_rect); // If we overflow, don't do so visibly (#4475)

        let frame = frame.unwrap_or_else(|| Frame::side_top_panel(ui.style()));
        let inner_response = frame.show(&mut panel_ui, |ui| {
            ui.set_min_height(ui.max_rect().height()); // Make sure the frame fills the full height
            ui.set_min_width((width_range.min - frame.inner_margin.sum().x).at_least(0.0));
            add_contents(ui)
        });

        let rect = inner_response.response.rect;

        {
            let mut cursor = ui.cursor();
            match side {
                Side::Left => {
                    cursor.min.x = rect.max.x;
                }
                Side::Right => {
                    cursor.max.x = rect.min.x;
                }
            }
            ui.set_cursor(cursor);
        }
        ui.expand_to_include_rect(rect);

        if resizable {
            // Now we do the actual resize interaction, on top of all the contents.
            // Otherwise its input could be eaten by the contents, e.g. a
            // `ScrollArea` on either side of the panel boundary.
            let resize_x = side.opposite().side_x(panel_rect);
            let resize_rect = Rect::from_x_y_ranges(resize_x..=resize_x, panel_rect.y_range())
                .expand2(vec2(ui.style().interaction.resize_grab_radius_side, 0.0));
            let resize_response = ui.interact(resize_rect, resize_id, Sense::drag());
            resize_hover = resize_response.hovered();
            is_resizing = resize_response.dragged();
        }

        if resize_hover || is_resizing {
            let cursor_icon = if width <= width_range.min {
                match self.side {
                    Side::Left => CursorIcon::ResizeEast,
                    Side::Right => CursorIcon::ResizeWest,
                }
            } else if width < width_range.max {
                CursorIcon::ResizeHorizontal
            } else {
                match self.side {
                    Side::Left => CursorIcon::ResizeWest,
                    Side::Right => CursorIcon::ResizeEast,
                }
            };
            ui.ctx().set_cursor_icon(cursor_icon);
        }

        // Keep this rect snapped so that panel content can be pixel-perfect
        let rect = ui.painter().round_rect_to_pixels(rect);

        PanelState { rect }.store(ui.ctx(), id);

        {
            let stroke = if is_resizing {
                ui.style().visuals.widgets.active.fg_stroke // highly visible
            } else if resize_hover {
                ui.style().visuals.widgets.hovered.fg_stroke // highly visible
            } else if show_separator_line {
                // TODO(emilk): distinguish resizable from non-resizable
                ui.style().visuals.widgets.noninteractive.bg_stroke // dim
            } else {
                Stroke::NONE
            };
            // TODO(emilk): draw line on top of all panels in this ui when https://github.com/emilk/egui/issues/1516 is done
            let resize_x = side.opposite().side_x(rect);

            // This makes it pixel-perfect for odd-sized strokes (width=1.0, width=3.0, etc)
            let resize_x = ui.painter().round_to_pixel_center(resize_x);

            // We want the line exactly on the last pixel but rust rounds away from zero so we bring it back a bit for
            // left-side panels
            let resize_x = resize_x - if side == Side::Left { 1.0 } else { 0.0 };
            ui.painter().vline(resize_x, panel_rect.y_range(), stroke);
        }

        inner_response
    }

    /// Show the panel at the top level.
    pub fn show<R>(
        self,
        ctx: &Context,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.show_dyn(ctx, Box::new(add_contents))
    }

    /// Show the panel at the top level.
    fn show_dyn<'c, R>(
        self,
        ctx: &Context,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let layer_id = LayerId::background();
        let side = self.side;
        let available_rect = ctx.available_rect();
        let mut panel_ui = Ui::new(
            ctx.clone(),
            layer_id,
            self.id,
            UiBuilder::new().max_rect(available_rect),
        );
        panel_ui.set_clip_rect(ctx.screen_rect());

        let inner_response = self.show_inside_dyn(&mut panel_ui, add_contents);
        let rect = inner_response.response.rect;

        match side {
            Side::Left => ctx.pass_state_mut(|state| {
                state.allocate_left_panel(Rect::from_min_max(available_rect.min, rect.max));
            }),
            Side::Right => ctx.pass_state_mut(|state| {
                state.allocate_right_panel(Rect::from_min_max(rect.min, available_rect.max));
            }),
        }
        inner_response
    }

    /// Show the panel if `is_expanded` is `true`,
    /// otherwise don't show it, but with a nice animation between collapsed and expanded.
    pub fn show_animated<R>(
        self,
        ctx: &Context,
        is_expanded: bool,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> Option<InnerResponse<R>> {
        let how_expanded = animate_expansion(ctx, self.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            None
        } else if how_expanded < 1.0 {
            // Show a fake panel in this in-between animation state:
            // TODO(emilk): move the panel out-of-screen instead of changing its width.
            // Then we can actually paint it as it animates.
            let expanded_width = PanelState::load(ctx, self.id)
                .map_or(self.default_width, |state| state.rect.width());
            let fake_width = how_expanded * expanded_width;
            Self {
                id: self.id.with("animating_panel"),
                ..self
            }
            .resizable(false)
            .exact_width(fake_width)
            .show(ctx, |_ui| {});
            None
        } else {
            // Show the real panel:
            Some(self.show(ctx, add_contents))
        }
    }

    /// Show the panel if `is_expanded` is `true`,
    /// otherwise don't show it, but with a nice animation between collapsed and expanded.
    pub fn show_animated_inside<R>(
        self,
        ui: &mut Ui,
        is_expanded: bool,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> Option<InnerResponse<R>> {
        let how_expanded = animate_expansion(ui.ctx(), self.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            None
        } else if how_expanded < 1.0 {
            // Show a fake panel in this in-between animation state:
            // TODO(emilk): move the panel out-of-screen instead of changing its width.
            // Then we can actually paint it as it animates.
            let expanded_width = PanelState::load(ui.ctx(), self.id)
                .map_or(self.default_width, |state| state.rect.width());
            let fake_width = how_expanded * expanded_width;
            Self {
                id: self.id.with("animating_panel"),
                ..self
            }
            .resizable(false)
            .exact_width(fake_width)
            .show_inside(ui, |_ui| {});
            None
        } else {
            // Show the real panel:
            Some(self.show_inside(ui, add_contents))
        }
    }

    /// Show either a collapsed or a expanded panel, with a nice animation between.
    pub fn show_animated_between<R>(
        ctx: &Context,
        is_expanded: bool,
        collapsed_panel: Self,
        expanded_panel: Self,
        add_contents: impl FnOnce(&mut Ui, f32) -> R,
    ) -> Option<InnerResponse<R>> {
        let how_expanded = animate_expansion(ctx, expanded_panel.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            Some(collapsed_panel.show(ctx, |ui| add_contents(ui, how_expanded)))
        } else if how_expanded < 1.0 {
            // Show animation:
            let collapsed_width = PanelState::load(ctx, collapsed_panel.id)
                .map_or(collapsed_panel.default_width, |state| state.rect.width());
            let expanded_width = PanelState::load(ctx, expanded_panel.id)
                .map_or(expanded_panel.default_width, |state| state.rect.width());
            let fake_width = lerp(collapsed_width..=expanded_width, how_expanded);
            Self {
                id: expanded_panel.id.with("animating_panel"),
                ..expanded_panel
            }
            .resizable(false)
            .exact_width(fake_width)
            .show(ctx, |ui| add_contents(ui, how_expanded));
            None
        } else {
            Some(expanded_panel.show(ctx, |ui| add_contents(ui, how_expanded)))
        }
    }

    /// Show either a collapsed or a expanded panel, with a nice animation between.
    pub fn show_animated_between_inside<R>(
        ui: &mut Ui,
        is_expanded: bool,
        collapsed_panel: Self,
        expanded_panel: Self,
        add_contents: impl FnOnce(&mut Ui, f32) -> R,
    ) -> InnerResponse<R> {
        let how_expanded =
            animate_expansion(ui.ctx(), expanded_panel.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            collapsed_panel.show_inside(ui, |ui| add_contents(ui, how_expanded))
        } else if how_expanded < 1.0 {
            // Show animation:
            let collapsed_width = PanelState::load(ui.ctx(), collapsed_panel.id)
                .map_or(collapsed_panel.default_width, |state| state.rect.width());
            let expanded_width = PanelState::load(ui.ctx(), expanded_panel.id)
                .map_or(expanded_panel.default_width, |state| state.rect.width());
            let fake_width = lerp(collapsed_width..=expanded_width, how_expanded);
            Self {
                id: expanded_panel.id.with("animating_panel"),
                ..expanded_panel
            }
            .resizable(false)
            .exact_width(fake_width)
            .show_inside(ui, |ui| add_contents(ui, how_expanded))
        } else {
            expanded_panel.show_inside(ui, |ui| add_contents(ui, how_expanded))
        }
    }
}

// ----------------------------------------------------------------------------

/// [`Top`](TopBottomSide::Top) or [`Bottom`](TopBottomSide::Bottom)
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum TopBottomSide {
    Top,
    Bottom,
}

impl TopBottomSide {
    fn opposite(self) -> Self {
        match self {
            Self::Top => Self::Bottom,
            Self::Bottom => Self::Top,
        }
    }

    fn set_rect_height(self, rect: &mut Rect, height: f32) {
        match self {
            Self::Top => rect.max.y = rect.min.y + height,
            Self::Bottom => rect.min.y = rect.max.y - height,
        }
    }

    fn side_y(self, rect: Rect) -> f32 {
        match self {
            Self::Top => rect.top(),
            Self::Bottom => rect.bottom(),
        }
    }
}

/// A panel that covers the entire top or bottom of a [`Ui`] or screen.
///
/// The order in which you add panels matter!
/// The first panel you add will always be the outermost, and the last you add will always be the innermost.
///
/// ⚠ Always add any [`CentralPanel`] last.
///
/// See the [module level docs](crate::containers::panel) for more details.
///
/// ```
/// # egui::__run_test_ctx(|ctx| {
/// egui::TopBottomPanel::top("my_panel").show(ctx, |ui| {
///    ui.label("Hello World!");
/// });
/// # });
/// ```
///
/// See also [`SidePanel`].
#[must_use = "You should call .show()"]
pub struct TopBottomPanel {
    side: TopBottomSide,
    id: Id,
    frame: Option<Frame>,
    resizable: bool,
    show_separator_line: bool,
    default_height: Option<f32>,
    height_range: Rangef,
}

impl TopBottomPanel {
    /// The id should be globally unique, e.g. `Id::new("my_top_panel")`.
    pub fn top(id: impl Into<Id>) -> Self {
        Self::new(TopBottomSide::Top, id)
    }

    /// The id should be globally unique, e.g. `Id::new("my_bottom_panel")`.
    pub fn bottom(id: impl Into<Id>) -> Self {
        Self::new(TopBottomSide::Bottom, id)
    }

    /// The id should be globally unique, e.g. `Id::new("my_panel")`.
    pub fn new(side: TopBottomSide, id: impl Into<Id>) -> Self {
        Self {
            side,
            id: id.into(),
            frame: None,
            resizable: false,
            show_separator_line: true,
            default_height: None,
            height_range: Rangef::new(20.0, f32::INFINITY),
        }
    }

    /// Can panel be resized by dragging the edge of it?
    ///
    /// Default is `false`.
    ///
    /// If you want your panel to be resizable you also need a widget in it that
    /// takes up more space as you resize it, such as:
    /// * Wrapping text ([`Ui::horizontal_wrapped`]).
    /// * A [`crate::ScrollArea`].
    /// * A [`crate::Separator`].
    /// * A [`crate::TextEdit`].
    /// * …
    #[inline]
    pub fn resizable(mut self, resizable: bool) -> Self {
        self.resizable = resizable;
        self
    }

    /// Show a separator line, even when not interacting with it?
    ///
    /// Default: `true`.
    #[inline]
    pub fn show_separator_line(mut self, show_separator_line: bool) -> Self {
        self.show_separator_line = show_separator_line;
        self
    }

    /// The initial height of the [`TopBottomPanel`], including margins.
    /// Defaults to [`crate::style::Spacing::interact_size`].y, plus frame margins.
    #[inline]
    pub fn default_height(mut self, default_height: f32) -> Self {
        self.default_height = Some(default_height);
        self.height_range = Rangef::new(
            self.height_range.min.at_most(default_height),
            self.height_range.max.at_least(default_height),
        );
        self
    }

    /// Minimum height of the panel, including margins.
    #[inline]
    pub fn min_height(mut self, min_height: f32) -> Self {
        self.height_range = Rangef::new(min_height, self.height_range.max.at_least(min_height));
        self
    }

    /// Maximum height of the panel, including margins.
    #[inline]
    pub fn max_height(mut self, max_height: f32) -> Self {
        self.height_range = Rangef::new(self.height_range.min.at_most(max_height), max_height);
        self
    }

    /// The allowable height range for the panel, including margins.
    #[inline]
    pub fn height_range(mut self, height_range: impl Into<Rangef>) -> Self {
        let height_range = height_range.into();
        self.default_height = self
            .default_height
            .map(|default_height| clamp_to_range(default_height, height_range));
        self.height_range = height_range;
        self
    }

    /// Enforce this exact height, including margins.
    #[inline]
    pub fn exact_height(mut self, height: f32) -> Self {
        self.default_height = Some(height);
        self.height_range = Rangef::point(height);
        self
    }

    /// Change the background color, margins, etc.
    #[inline]
    pub fn frame(mut self, frame: Frame) -> Self {
        self.frame = Some(frame);
        self
    }
}

impl TopBottomPanel {
    /// Show the panel inside a [`Ui`].
    pub fn show_inside<R>(
        self,
        ui: &mut Ui,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.show_inside_dyn(ui, Box::new(add_contents))
    }

    /// Show the panel inside a [`Ui`].
    fn show_inside_dyn<'c, R>(
        self,
        ui: &mut Ui,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let Self {
            side,
            id,
            frame,
            resizable,
            show_separator_line,
            default_height,
            height_range,
        } = self;

        let frame = frame.unwrap_or_else(|| Frame::side_top_panel(ui.style()));

        let available_rect = ui.available_rect_before_wrap();
        let mut panel_rect = available_rect;

        let mut height = if let Some(state) = PanelState::load(ui.ctx(), id) {
            state.rect.height()
        } else {
            default_height
                .unwrap_or_else(|| ui.style().spacing.interact_size.y + frame.inner_margin.sum().y)
        };
        {
            height = clamp_to_range(height, height_range).at_most(available_rect.height());
            side.set_rect_height(&mut panel_rect, height);
            ui.ctx()
                .check_for_id_clash(id, panel_rect, "TopBottomPanel");
        }

        let resize_id = id.with("__resize");
        let mut resize_hover = false;
        let mut is_resizing = false;
        if resizable {
            // First we read the resize interaction results, to avoid frame latency in the resize:
            if let Some(resize_response) = ui.ctx().read_response(resize_id) {
                resize_hover = resize_response.hovered();
                is_resizing = resize_response.dragged();

                if is_resizing {
                    if let Some(pointer) = resize_response.interact_pointer_pos() {
                        height = (pointer.y - side.side_y(panel_rect)).abs();
                        height =
                            clamp_to_range(height, height_range).at_most(available_rect.height());
                        side.set_rect_height(&mut panel_rect, height);
                    }
                }
            }
        }

        let mut panel_ui = ui.new_child(
            UiBuilder::new()
                .id_salt(id)
                .ui_stack_info(UiStackInfo::new(match side {
                    TopBottomSide::Top => UiKind::TopPanel,
                    TopBottomSide::Bottom => UiKind::BottomPanel,
                }))
                .max_rect(panel_rect)
                .layout(Layout::top_down(Align::Min)),
        );
        panel_ui.expand_to_include_rect(panel_rect);
        panel_ui.set_clip_rect(panel_rect); // If we overflow, don't do so visibly (#4475)

        let inner_response = frame.show(&mut panel_ui, |ui| {
            ui.set_min_width(ui.max_rect().width()); // Make the frame fill full width
            ui.set_min_height((height_range.min - frame.inner_margin.sum().y).at_least(0.0));
            add_contents(ui)
        });

        let rect = inner_response.response.rect;

        {
            let mut cursor = ui.cursor();
            match side {
                TopBottomSide::Top => {
                    cursor.min.y = rect.max.y;
                }
                TopBottomSide::Bottom => {
                    cursor.max.y = rect.min.y;
                }
            }
            ui.set_cursor(cursor);
        }
        ui.expand_to_include_rect(rect);

        if resizable {
            // Now we do the actual resize interaction, on top of all the contents.
            // Otherwise its input could be eaten by the contents, e.g. a
            // `ScrollArea` on either side of the panel boundary.

            let resize_y = side.opposite().side_y(panel_rect);
            let resize_rect = Rect::from_x_y_ranges(panel_rect.x_range(), resize_y..=resize_y)
                .expand2(vec2(0.0, ui.style().interaction.resize_grab_radius_side));
            let resize_response = ui.interact(resize_rect, resize_id, Sense::drag());
            resize_hover = resize_response.hovered();
            is_resizing = resize_response.dragged();
        }

        if resize_hover || is_resizing {
            let cursor_icon = if height <= height_range.min {
                match self.side {
                    TopBottomSide::Top => CursorIcon::ResizeSouth,
                    TopBottomSide::Bottom => CursorIcon::ResizeNorth,
                }
            } else if height < height_range.max {
                CursorIcon::ResizeVertical
            } else {
                match self.side {
                    TopBottomSide::Top => CursorIcon::ResizeNorth,
                    TopBottomSide::Bottom => CursorIcon::ResizeSouth,
                }
            };
            ui.ctx().set_cursor_icon(cursor_icon);
        }

        // Keep this rect snapped so that panel content can be pixel-perfect
        let rect = ui.painter().round_rect_to_pixels(rect);

        PanelState { rect }.store(ui.ctx(), id);

        {
            let stroke = if is_resizing {
                ui.style().visuals.widgets.active.fg_stroke // highly visible
            } else if resize_hover {
                ui.style().visuals.widgets.hovered.fg_stroke // highly visible
            } else if show_separator_line {
                // TODO(emilk): distinguish resizable from non-resizable
                ui.style().visuals.widgets.noninteractive.bg_stroke // dim
            } else {
                Stroke::NONE
            };
            // TODO(emilk): draw line on top of all panels in this ui when https://github.com/emilk/egui/issues/1516 is done
            let resize_y = side.opposite().side_y(rect);
            let resize_y = ui.painter().round_to_pixel_center(resize_y);

            // We want the line exactly on the last pixel but rust rounds away from zero so we bring it back a bit for
            // top-side panels
            let resize_y = resize_y - if side == TopBottomSide::Top { 1.0 } else { 0.0 };
            ui.painter().hline(panel_rect.x_range(), resize_y, stroke);
        }

        inner_response
    }

    /// Show the panel at the top level.
    pub fn show<R>(
        self,
        ctx: &Context,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.show_dyn(ctx, Box::new(add_contents))
    }

    /// Show the panel at the top level.
    fn show_dyn<'c, R>(
        self,
        ctx: &Context,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let layer_id = LayerId::background();
        let available_rect = ctx.available_rect();
        let side = self.side;

        let mut panel_ui = Ui::new(
            ctx.clone(),
            layer_id,
            self.id,
            UiBuilder::new().max_rect(available_rect),
        );
        panel_ui.set_clip_rect(ctx.screen_rect());

        let inner_response = self.show_inside_dyn(&mut panel_ui, add_contents);
        let rect = inner_response.response.rect;

        match side {
            TopBottomSide::Top => {
                ctx.pass_state_mut(|state| {
                    state.allocate_top_panel(Rect::from_min_max(available_rect.min, rect.max));
                });
            }
            TopBottomSide::Bottom => {
                ctx.pass_state_mut(|state| {
                    state.allocate_bottom_panel(Rect::from_min_max(rect.min, available_rect.max));
                });
            }
        }

        inner_response
    }

    /// Show the panel if `is_expanded` is `true`,
    /// otherwise don't show it, but with a nice animation between collapsed and expanded.
    pub fn show_animated<R>(
        self,
        ctx: &Context,
        is_expanded: bool,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> Option<InnerResponse<R>> {
        let how_expanded = animate_expansion(ctx, self.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            None
        } else if how_expanded < 1.0 {
            // Show a fake panel in this in-between animation state:
            // TODO(emilk): move the panel out-of-screen instead of changing its height.
            // Then we can actually paint it as it animates.
            let expanded_height = PanelState::load(ctx, self.id)
                .map(|state| state.rect.height())
                .or(self.default_height)
                .unwrap_or_else(|| ctx.style().spacing.interact_size.y);
            let fake_height = how_expanded * expanded_height;
            Self {
                id: self.id.with("animating_panel"),
                ..self
            }
            .resizable(false)
            .exact_height(fake_height)
            .show(ctx, |_ui| {});
            None
        } else {
            // Show the real panel:
            Some(self.show(ctx, add_contents))
        }
    }

    /// Show the panel if `is_expanded` is `true`,
    /// otherwise don't show it, but with a nice animation between collapsed and expanded.
    pub fn show_animated_inside<R>(
        self,
        ui: &mut Ui,
        is_expanded: bool,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> Option<InnerResponse<R>> {
        let how_expanded = animate_expansion(ui.ctx(), self.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            None
        } else if how_expanded < 1.0 {
            // Show a fake panel in this in-between animation state:
            // TODO(emilk): move the panel out-of-screen instead of changing its height.
            // Then we can actually paint it as it animates.
            let expanded_height = PanelState::load(ui.ctx(), self.id)
                .map(|state| state.rect.height())
                .or(self.default_height)
                .unwrap_or_else(|| ui.style().spacing.interact_size.y);
            let fake_height = how_expanded * expanded_height;
            Self {
                id: self.id.with("animating_panel"),
                ..self
            }
            .resizable(false)
            .exact_height(fake_height)
            .show_inside(ui, |_ui| {});
            None
        } else {
            // Show the real panel:
            Some(self.show_inside(ui, add_contents))
        }
    }

    /// Show either a collapsed or a expanded panel, with a nice animation between.
    pub fn show_animated_between<R>(
        ctx: &Context,
        is_expanded: bool,
        collapsed_panel: Self,
        expanded_panel: Self,
        add_contents: impl FnOnce(&mut Ui, f32) -> R,
    ) -> Option<InnerResponse<R>> {
        let how_expanded = animate_expansion(ctx, expanded_panel.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            Some(collapsed_panel.show(ctx, |ui| add_contents(ui, how_expanded)))
        } else if how_expanded < 1.0 {
            // Show animation:
            let collapsed_height = PanelState::load(ctx, collapsed_panel.id)
                .map(|state| state.rect.height())
                .or(collapsed_panel.default_height)
                .unwrap_or_else(|| ctx.style().spacing.interact_size.y);

            let expanded_height = PanelState::load(ctx, expanded_panel.id)
                .map(|state| state.rect.height())
                .or(expanded_panel.default_height)
                .unwrap_or_else(|| ctx.style().spacing.interact_size.y);

            let fake_height = lerp(collapsed_height..=expanded_height, how_expanded);
            Self {
                id: expanded_panel.id.with("animating_panel"),
                ..expanded_panel
            }
            .resizable(false)
            .exact_height(fake_height)
            .show(ctx, |ui| add_contents(ui, how_expanded));
            None
        } else {
            Some(expanded_panel.show(ctx, |ui| add_contents(ui, how_expanded)))
        }
    }

    /// Show either a collapsed or a expanded panel, with a nice animation between.
    pub fn show_animated_between_inside<R>(
        ui: &mut Ui,
        is_expanded: bool,
        collapsed_panel: Self,
        expanded_panel: Self,
        add_contents: impl FnOnce(&mut Ui, f32) -> R,
    ) -> InnerResponse<R> {
        let how_expanded =
            animate_expansion(ui.ctx(), expanded_panel.id.with("animation"), is_expanded);

        if 0.0 == how_expanded {
            collapsed_panel.show_inside(ui, |ui| add_contents(ui, how_expanded))
        } else if how_expanded < 1.0 {
            // Show animation:
            let collapsed_height = PanelState::load(ui.ctx(), collapsed_panel.id)
                .map(|state| state.rect.height())
                .or(collapsed_panel.default_height)
                .unwrap_or_else(|| ui.style().spacing.interact_size.y);

            let expanded_height = PanelState::load(ui.ctx(), expanded_panel.id)
                .map(|state| state.rect.height())
                .or(expanded_panel.default_height)
                .unwrap_or_else(|| ui.style().spacing.interact_size.y);

            let fake_height = lerp(collapsed_height..=expanded_height, how_expanded);
            Self {
                id: expanded_panel.id.with("animating_panel"),
                ..expanded_panel
            }
            .resizable(false)
            .exact_height(fake_height)
            .show_inside(ui, |ui| add_contents(ui, how_expanded))
        } else {
            expanded_panel.show_inside(ui, |ui| add_contents(ui, how_expanded))
        }
    }
}

// ----------------------------------------------------------------------------

/// A panel that covers the remainder of the screen,
/// i.e. whatever area is left after adding other panels.
///
/// The order in which you add panels matter!
/// The first panel you add will always be the outermost, and the last you add will always be the innermost.
///
/// ⚠ [`CentralPanel`] must be added after all other panels!
///
/// NOTE: Any [`crate::Window`]s and [`crate::Area`]s will cover the top-level [`CentralPanel`].
///
/// See the [module level docs](crate::containers::panel) for more details.
///
/// ```
/// # egui::__run_test_ctx(|ctx| {
/// egui::TopBottomPanel::top("my_panel").show(ctx, |ui| {
///    ui.label("Hello World! From `TopBottomPanel`, that must be before `CentralPanel`!");
/// });
/// egui::CentralPanel::default().show(ctx, |ui| {
///    ui.label("Hello World!");
/// });
/// # });
/// ```
#[must_use = "You should call .show()"]
#[derive(Default)]
pub struct CentralPanel {
    frame: Option<Frame>,
}

impl CentralPanel {
    /// Change the background color, margins, etc.
    #[inline]
    pub fn frame(mut self, frame: Frame) -> Self {
        self.frame = Some(frame);
        self
    }
}

impl CentralPanel {
    /// Show the panel inside a [`Ui`].
    pub fn show_inside<R>(
        self,
        ui: &mut Ui,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.show_inside_dyn(ui, Box::new(add_contents))
    }

    /// Show the panel inside a [`Ui`].
    fn show_inside_dyn<'c, R>(
        self,
        ui: &mut Ui,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let Self { frame } = self;

        let panel_rect = ui.available_rect_before_wrap();
        let mut panel_ui = ui.new_child(
            UiBuilder::new()
                .ui_stack_info(UiStackInfo::new(UiKind::CentralPanel))
                .max_rect(panel_rect)
                .layout(Layout::top_down(Align::Min)),
        );
        panel_ui.set_clip_rect(panel_rect); // If we overflow, don't do so visibly (#4475)

        let frame = frame.unwrap_or_else(|| Frame::central_panel(ui.style()));
        frame.show(&mut panel_ui, |ui| {
            ui.expand_to_include_rect(ui.max_rect()); // Expand frame to include it all
            add_contents(ui)
        })
    }

    /// Show the panel at the top level.
    pub fn show<R>(
        self,
        ctx: &Context,
        add_contents: impl FnOnce(&mut Ui) -> R,
    ) -> InnerResponse<R> {
        self.show_dyn(ctx, Box::new(add_contents))
    }

    /// Show the panel at the top level.
    fn show_dyn<'c, R>(
        self,
        ctx: &Context,
        add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,
    ) -> InnerResponse<R> {
        let available_rect = ctx.available_rect();
        let layer_id = LayerId::background();
        let id = Id::new((ctx.viewport_id(), "central_panel"));

        let mut panel_ui = Ui::new(
            ctx.clone(),
            layer_id,
            id,
            UiBuilder::new().max_rect(available_rect),
        );
        panel_ui.set_clip_rect(ctx.screen_rect());

        let inner_response = self.show_inside_dyn(&mut panel_ui, add_contents);

        // Only inform ctx about what we actually used, so we can shrink the native window to fit.
        ctx.pass_state_mut(|state| state.allocate_central_panel(inner_response.response.rect));

        inner_response
    }
}

fn clamp_to_range(x: f32, range: Rangef) -> f32 {
    let range = range.as_positive();
    x.clamp(range.min, range.max)
}
