Fix `Ui::scroll_with_delta` only scrolling if the `ScrollArea` is focused (#4303)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to add commits to your PR.
* Remember to run `cargo fmt` and `cargo cranky`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

This introduces the boolean field force_current_scroll_area to
InputState which will be set when scroll_with_delta is called, causing
the ScrollArea to skip the check whether it is focused and always
consume the smooth scroll delta.

* Closes #2783 
* Related to #4295

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/containers/scroll_area.rs

@@ -783,7 +783,14 @@ impl Prepared {
 
         let content_size = content_ui.min_size();
 
+        let scroll_delta = content_ui
+            .ctx()
+            .frame_state_mut(|state| std::mem::take(&mut state.scroll_delta));
+
         for d in 0..2 {
+            // FrameState::scroll_delta is inverted from the way we apply the delta, so we need to negate it.
+            let mut delta = -scroll_delta[d];
+
             // We always take both scroll targets regardless of which scroll axes are enabled. This
             // is to avoid them leaking to other scroll areas.
             let scroll_target = content_ui
@@ -791,7 +798,7 @@ impl Prepared {
                 .frame_state_mut(|state| state.scroll_target[d].take());
 
             if scroll_enabled[d] {
-                if let Some((target_range, align)) = scroll_target {
+                delta += if let Some((target_range, align)) = scroll_target {
                     let min = content_ui.min_rect().min[d];
                     let clip_rect = content_ui.clip_rect();
                     let visible_range = min..=min + clip_rect.size()[d];
@@ -800,7 +807,7 @@ impl Prepared {
                     let clip_end = clip_rect.max[d];
                     let mut spacing = ui.spacing().item_spacing[d];
 
-                    let delta = if let Some(align) = align {
+                    if let Some(align) = align {
                         let center_factor = align.to_factor();
 
                         let offset =
@@ -817,31 +824,32 @@ impl Prepared {
                     } else {
                         // Ui is already in view, no need to adjust scroll.
                         0.0
-                    };
-
-                    if delta != 0.0 {
-                        let target_offset = state.offset[d] + delta;
-
-                        if !animated {
-                            state.offset[d] = target_offset;
-                        } else if let Some(animation) = &mut state.offset_target[d] {
-                            // For instance: the user is continuously calling `ui.scroll_to_cursor`,
-                            // so we don't want to reset the animation, but perhaps update the target:
-                            animation.target_offset = target_offset;
-                        } else {
-                            // The further we scroll, the more time we take.
-                            // TODO(emilk): let users configure this in `Style`.
-                            let now = ui.input(|i| i.time);
-                            let points_per_second = 1000.0;
-                            let animation_duration =
-                                (delta.abs() / points_per_second).clamp(0.1, 0.3);
-                            state.offset_target[d] = Some(ScrollTarget {
-                                animation_time_span: (now, now + animation_duration as f64),
-                                target_offset,
-                            });
-                        }
-                        ui.ctx().request_repaint();
                     }
+                } else {
+                    0.0
+                };
+
+                if delta != 0.0 {
+                    let target_offset = state.offset[d] + delta;
+
+                    if !animated {
+                        state.offset[d] = target_offset;
+                    } else if let Some(animation) = &mut state.offset_target[d] {
+                        // For instance: the user is continuously calling `ui.scroll_to_cursor`,
+                        // so we don't want to reset the animation, but perhaps update the target:
+                        animation.target_offset = target_offset;
+                    } else {
+                        // The further we scroll, the more time we take.
+                        // TODO(emilk): let users configure this in `Style`.
+                        let now = ui.input(|i| i.time);
+                        let points_per_second = 1000.0;
+                        let animation_duration = (delta.abs() / points_per_second).clamp(0.1, 0.3);
+                        state.offset_target[d] = Some(ScrollTarget {
+                            animation_time_span: (now, now + animation_duration as f64),
+                            target_offset,
+                        });
+                    }
+                    ui.ctx().request_repaint();
                 }
             }
         }

crates/egui/src/frame_state.rs

@@ -38,9 +38,20 @@ pub(crate) struct FrameState {
     /// Initialized to `None` at the start of each frame.
     pub(crate) tooltip_state: Option<TooltipFrameState>,
 
-    /// horizontal, vertical
+    /// The current scroll area should scroll to this range (horizontal, vertical).
     pub(crate) scroll_target: [Option<(Rangef, Option<Align>)>; 2],
 
+    /// The current scroll area should scroll by this much.
+    ///
+    /// The delta dictates how the _content_ should move.
+    ///
+    /// A positive X-value indicates the content is being moved right,
+    /// as when swiping right on a touch-screen or track-pad with natural scrolling.
+    ///
+    /// A positive Y-value indicates the content is being moved down,
+    /// as when swiping down on a touch-screen or track-pad with natural scrolling.
+    pub(crate) scroll_delta: Vec2,
+
     #[cfg(feature = "accesskit")]
     pub(crate) accesskit_state: Option<AccessKitFrameState>,
 
@@ -63,6 +74,7 @@ impl Default for FrameState {
             used_by_panels: Rect::NAN,
             tooltip_state: None,
             scroll_target: [None, None],
+            scroll_delta: Vec2::default(),
             #[cfg(feature = "accesskit")]
             accesskit_state: None,
             highlight_this_frame: Default::default(),
@@ -84,6 +96,7 @@ impl FrameState {
             used_by_panels,
             tooltip_state,
             scroll_target,
+            scroll_delta,
             #[cfg(feature = "accesskit")]
             accesskit_state,
             highlight_this_frame,
@@ -99,6 +112,7 @@ impl FrameState {
         *used_by_panels = Rect::NOTHING;
         *tooltip_state = None;
         *scroll_target = [None, None];
+        *scroll_delta = Vec2::default();
 
         #[cfg(debug_assertions)]
         {

crates/egui/src/ui.rs

@@ -1075,6 +1075,8 @@ impl Ui {
     /// A positive Y-value indicates the content is being moved down,
     /// as when swiping down on a touch-screen or track-pad with natural scrolling.
     ///
+    /// If this is called multiple times per frame for the same [`ScrollArea`], the deltas will be summed.
+    ///
     /// /// See also: [`Response::scroll_to_me`], [`Ui::scroll_to_rect`], [`Ui::scroll_to_cursor`]
     ///
     /// ```
@@ -1093,8 +1095,9 @@ impl Ui {
     /// # });
     /// ```
     pub fn scroll_with_delta(&self, delta: Vec2) {
-        self.ctx()
+        self.ctx().frame_state_mut(|state| {
-            .input_mut(|input| input.smooth_scroll_delta += delta);
+            state.scroll_delta += delta;
+        });
     }
 }
 

crates/egui_demo_lib/src/demo/scrolling.rs

@@ -236,6 +236,7 @@ struct ScrollTo {
     track_item: usize,
     tack_item_align: Option<Align>,
     offset: f32,
+    delta: f32,
 }
 
 impl Default for ScrollTo {
@@ -244,6 +245,7 @@ impl Default for ScrollTo {
             track_item: 25,
             tack_item_align: Some(Align::Center),
             offset: 0.0,
+            delta: 64.0,
         }
     }
 }
@@ -258,6 +260,7 @@ impl super::View for ScrollTo {
         let mut go_to_scroll_offset = false;
         let mut scroll_top = false;
         let mut scroll_bottom = false;
+        let mut scroll_delta = None;
 
         ui.horizontal(|ui| {
             ui.label("Scroll to a specific item index:");
@@ -294,6 +297,20 @@ impl super::View for ScrollTo {
             scroll_bottom |= ui.button("Scroll to bottom").clicked();
         });
 
+        ui.horizontal(|ui| {
+            ui.label("Scroll by");
+            DragValue::new(&mut self.delta)
+                .speed(1.0)
+                .suffix("px")
+                .ui(ui);
+            if ui.button("⬇").clicked() {
+                scroll_delta = Some(self.delta * Vec2::UP); // scroll down (move contents up)
+            }
+            if ui.button("⬆").clicked() {
+                scroll_delta = Some(self.delta * Vec2::DOWN); // scroll up (move contents down)
+            }
+        });
+
         let mut scroll_area = ScrollArea::vertical().max_height(200.0).auto_shrink(false);
         if go_to_scroll_offset {
             scroll_area = scroll_area.vertical_scroll_offset(self.offset);
@@ -305,6 +322,10 @@ impl super::View for ScrollTo {
                 if scroll_top {
                     ui.scroll_to_cursor(Some(Align::TOP));
                 }
+                if let Some(scroll_delta) = scroll_delta {
+                    ui.scroll_with_delta(scroll_delta);
+                }
+
                 ui.vertical(|ui| {
                     for item in 1..=num_items {
                         if track_item && item == self.track_item {