gpui: Fix slow scrolling in lists (#33608)

matches editor element's behavior


https://github.com/user-attachments/assets/f70912e1-5adb-403b-a98c-63e2e89929ac


- in first version editor scrolls like 1.5 pages, but agent panel only
scrolls half a page.
- in second version, agent panel also scrolls like 1.5 pages.

Release Notes:

- Fixed skipping of some scroll events in the non-uniform list UI element, which fixes slow scrolling of the agent panel.

crates/gpui/src/elements/list.rs

@@ -10,8 +10,8 @@
 use crate::{
     AnyElement, App, AvailableSpace, Bounds, ContentMask, DispatchPhase, Edges, Element, EntityId,
     FocusHandle, GlobalElementId, Hitbox, HitboxBehavior, InspectorElementId, IntoElement,
-    Overflow, Pixels, Point, ScrollWheelEvent, Size, Style, StyleRefinement, Styled, Window, point,
-    px, size,
+    Overflow, Pixels, Point, ScrollDelta, ScrollWheelEvent, Size, Style, StyleRefinement, Styled,
+    Window, point, px, size,
 };
 use collections::VecDeque;
 use refineable::Refineable as _;
@@ -962,12 +962,15 @@ impl Element for List {
         let height = bounds.size.height;
         let scroll_top = prepaint.layout.scroll_top;
         let hitbox_id = prepaint.hitbox.id;
+        let mut accumulated_scroll_delta = ScrollDelta::default();
         window.on_mouse_event(move |event: &ScrollWheelEvent, phase, window, cx| {
             if phase == DispatchPhase::Bubble && hitbox_id.should_handle_scroll(window) {
+                accumulated_scroll_delta = accumulated_scroll_delta.coalesce(event.delta);
+                let pixel_delta = accumulated_scroll_delta.pixel_delta(px(20.));
                 list_state.0.borrow_mut().scroll(
                     &scroll_top,
                     height,
-                    event.delta.pixel_delta(px(20.)),
+                    pixel_delta,
                     current_view,
                     window,
                     cx,