Smooth scrolling (#3884)

This adds smooth scrolling in egui. This makes scrolling in a
`ScrollArea` using a notched mouse wheel a lot nicer.

`InputState::scroll_delta` has been replaced by
`InputState::raw_scroll_delta` and `InputState::smooth_scroll_delta`.

crates/egui/src/containers/scroll_area.rs

@@ -564,6 +564,7 @@ impl ScrollArea {
                     }
                 }
             } else {
+                // Kinetic scrolling
                 let stop_speed = 20.0; // Pixels per second.
                 let friction_coeff = 1000.0; // Pixels per second squared.
                 let dt = ui.input(|i| i.unstable_dt);
@@ -781,12 +782,12 @@ impl Prepared {
                 && scroll_enabled[0] != scroll_enabled[1];
             for d in 0..2 {
                 if scroll_enabled[d] {
-                    let scroll_delta = ui.ctx().frame_state(|fs| {
+                    let scroll_delta = ui.ctx().input_mut(|input| {
                         if always_scroll_enabled_direction {
                             // no bidirectional scrolling; allow horizontal scrolling without pressing shift
-                            fs.scroll_delta[0] + fs.scroll_delta[1]
+                            input.smooth_scroll_delta[0] + input.smooth_scroll_delta[1]
                         } else {
-                            fs.scroll_delta[d]
+                            input.smooth_scroll_delta[d]
                         }
                     });
 
@@ -795,15 +796,17 @@ impl Prepared {
 
                     if scrolling_up || scrolling_down {
                         state.offset[d] -= scroll_delta;
-                        // Clear scroll delta so no parent scroll will use it.
+
-                        ui.ctx().frame_state_mut(|fs| {
+                        // Clear scroll delta so no parent scroll will use it:
+                        ui.ctx().input_mut(|input| {
                             if always_scroll_enabled_direction {
-                                fs.scroll_delta[0] = 0.0;
+                                input.smooth_scroll_delta[0] = 0.0;
-                                fs.scroll_delta[1] = 0.0;
+                                input.smooth_scroll_delta[1] = 0.0;
                             } else {
-                                fs.scroll_delta[d] = 0.0;
+                                input.smooth_scroll_delta[d] = 0.0;
                             }
                         });
+
                         state.scroll_stuck_to_end[d] = false;
                     }
                 }

crates/egui/src/frame_state.rs

@@ -38,11 +38,6 @@ pub(crate) struct FrameState {
     /// Initialized to `None` at the start of each frame.
     pub(crate) tooltip_state: Option<TooltipFrameState>,
 
-    /// Set to [`InputState::scroll_delta`] on the start of each frame.
-    ///
-    /// Cleared by the first [`ScrollArea`] that makes use of it.
-    pub(crate) scroll_delta: Vec2, // TODO(emilk): move to `InputState` ?
-
     /// horizontal, vertical
     pub(crate) scroll_target: [Option<(Rangef, Option<Align>)>; 2],
 
@@ -67,7 +62,6 @@ impl Default for FrameState {
             unused_rect: Rect::NAN,
             used_by_panels: Rect::NAN,
             tooltip_state: None,
-            scroll_delta: Vec2::ZERO,
             scroll_target: [None, None],
             #[cfg(feature = "accesskit")]
             accesskit_state: None,
@@ -89,7 +83,6 @@ impl FrameState {
             unused_rect,
             used_by_panels,
             tooltip_state,
-            scroll_delta,
             scroll_target,
             #[cfg(feature = "accesskit")]
             accesskit_state,
@@ -105,7 +98,6 @@ impl FrameState {
         *unused_rect = input.screen_rect();
         *used_by_panels = Rect::NOTHING;
         *tooltip_state = None;
-        *scroll_delta = input.scroll_delta;
         *scroll_target = [None, None];
 
         #[cfg(debug_assertions)]

crates/egui/src/input_state.rs

@@ -33,7 +33,10 @@ pub struct InputState {
     /// (We keep a separate [`TouchState`] for each encountered touch device.)
     touch_states: BTreeMap<TouchDeviceId, TouchState>,
 
-    /// How many points the user scrolled.
+    /// Used for smoothing the scroll delta.
+    unprocessed_scroll_delta: Vec2,
+
+    /// The raw input of how many points the user scrolled.
     ///
     /// The delta dictates how the _content_ should move.
     ///
@@ -42,7 +45,24 @@ pub struct InputState {
     ///
     /// 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 scroll_delta: Vec2,
+    ///
+    /// When using a notched scroll-wheel this will spike very large for one frame,
+    /// then drop to zero. For a smoother experience, use [`Self::smooth_scroll_delta`].
+    pub raw_scroll_delta: Vec2,
+
+    /// How many points the user scrolled, smoothed over a few frames.
+    ///
+    /// 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.
+    ///
+    /// [`crate::ScrollArea`] will both read and write to this field, so that
+    /// at the end of the frame this will be zero if a scroll-area consumed the delta.
+    pub smooth_scroll_delta: Vec2,
 
     /// Zoom scale factor this frame (e.g. from ctrl-scroll or pinch gesture).
     ///
@@ -125,7 +145,9 @@ impl Default for InputState {
             raw: Default::default(),
             pointer: Default::default(),
             touch_states: Default::default(),
-            scroll_delta: Vec2::ZERO,
+            unprocessed_scroll_delta: Vec2::ZERO,
+            raw_scroll_delta: Vec2::ZERO,
+            smooth_scroll_delta: Vec2::ZERO,
             zoom_factor_delta: 1.0,
             screen_rect: Rect::from_min_size(Default::default(), vec2(10_000.0, 10_000.0)),
             pixels_per_point: 1.0,
@@ -171,7 +193,7 @@ impl InputState {
         let pointer = self.pointer.begin_frame(time, &new);
 
         let mut keys_down = self.keys_down;
-        let mut scroll_delta = Vec2::ZERO;
+        let mut raw_scroll_delta = Vec2::ZERO;
         let mut zoom_factor_delta = 1.0;
         for event in &mut new.events {
             match event {
@@ -189,7 +211,7 @@ impl InputState {
                     }
                 }
                 Event::Scroll(delta) => {
-                    scroll_delta += *delta;
+                    raw_scroll_delta += *delta;
                 }
                 Event::Zoom(factor) => {
                     zoom_factor_delta *= *factor;
@@ -198,6 +220,22 @@ impl InputState {
             }
         }
 
+        let mut unprocessed_scroll_delta = self.unprocessed_scroll_delta;
+
+        let smooth_scroll_delta;
+
+        {
+            // Mouse wheels often go very large steps.
+            // A single notch on a logitech mouse wheel connected to a Macbook returns 14.0 raw_scroll_delta.
+            // So we smooth it out over several frames for a nicer user experience when scrolling in egui.
+            unprocessed_scroll_delta += raw_scroll_delta;
+            let dt = stable_dt.at_most(0.1);
+            let t = crate::emath::exponential_smooth_factor(0.90, 0.1, dt); // reach _% in _ seconds. TODO: parameterize
+
+            smooth_scroll_delta = t * unprocessed_scroll_delta;
+            unprocessed_scroll_delta -= smooth_scroll_delta;
+        }
+
         let mut modifiers = new.modifiers;
 
         let focused_changed = self.focused != new.focused
@@ -215,7 +253,9 @@ impl InputState {
         Self {
             pointer,
             touch_states: self.touch_states,
-            scroll_delta,
+            unprocessed_scroll_delta,
+            raw_scroll_delta,
+            smooth_scroll_delta,
             zoom_factor_delta,
             screen_rect,
             pixels_per_point,
@@ -282,8 +322,11 @@ impl InputState {
         )
     }
 
+    /// The [`crate::Context`] will call this at the end of each frame to see if we need a repaint.
     pub fn wants_repaint(&self) -> bool {
-        self.pointer.wants_repaint() || self.scroll_delta != Vec2::ZERO || !self.events.is_empty()
+        self.pointer.wants_repaint()
+            || self.unprocessed_scroll_delta.abs().max_elem() > 0.2
+            || !self.events.is_empty()
     }
 
     /// Count presses of a key. If non-zero, the presses are consumed, so that this will only return non-zero once.
@@ -1007,7 +1050,11 @@ impl InputState {
             raw,
             pointer,
             touch_states,
-            scroll_delta,
+
+            unprocessed_scroll_delta,
+            raw_scroll_delta,
+            smooth_scroll_delta,
+
             zoom_factor_delta,
             screen_rect,
             pixels_per_point,
@@ -1042,7 +1089,15 @@ impl InputState {
             });
         }
 
-        ui.label(format!("scroll_delta: {scroll_delta:?} points"));
+        if cfg!(debug_assertions) {
+            ui.label(format!(
+                "unprocessed_scroll_delta: {unprocessed_scroll_delta:?} points"
+            ));
+        }
+        ui.label(format!("raw_scroll_delta: {raw_scroll_delta:?} points"));
+        ui.label(format!(
+            "smooth_scroll_delta: {smooth_scroll_delta:?} points"
+        ));
         ui.label(format!("zoom_factor_delta: {zoom_factor_delta:4.2}x"));
         ui.label(format!("screen_rect: {screen_rect:?} points"));
         ui.label(format!(

crates/egui/src/ui.rs

@@ -1042,7 +1042,7 @@ impl Ui {
     /// ```
     pub fn scroll_with_delta(&self, delta: Vec2) {
         self.ctx()
-            .frame_state_mut(|state| state.scroll_delta += delta);
+            .input_mut(|input| input.smooth_scroll_delta += delta);
     }
 }
 

crates/egui_plot/src/lib.rs

@@ -1115,7 +1115,7 @@ impl Plot {
                 }
             }
             if allow_scroll {
-                let scroll_delta = ui.input(|i| i.scroll_delta);
+                let scroll_delta = ui.input(|i| i.smooth_scroll_delta);
                 if scroll_delta != Vec2::ZERO {
                     transform.translate_bounds(-scroll_delta);
                     auto_bounds = false.into();