Update ScrollArea drag velocity when drag stopped

Fixes #5174. The drag velocity was not being updated unless the cursor
counted as "dragging", which only happens when it's in motion. This
effectively guarantees that the drag velocity will never be zero, even
if the cursor is not moving, and results in spurious scroll velocity
being applied when the cursor is released. Instead, we update the
velocity only when the drag is stopped, which is when the kinetic
scrolling actually needs to begin.

crates/egui/src/containers/scroll_area.rs

@@ -621,20 +621,35 @@ impl ScrollArea {
                 .interact_rect
                 .map(|rect| ui.interact(rect, id.with("area"), Sense::drag()));
 
-            if content_response_option.map(|response| response.dragged()) == Some(true) {
+            if content_response_option
+                .as_ref()
+                .is_some_and(|response| response.dragged())
+            {
                 for d in 0..2 {
                     if scroll_enabled[d] {
                         ui.input(|input| {
                             state.offset[d] -= input.pointer.delta()[d];
-                            state.vel[d] = input.pointer.velocity()[d];
                         });
                         state.scroll_stuck_to_end[d] = false;
                         state.offset_target[d] = None;
-                    } else {
-                        state.vel[d] = 0.0;
                     }
                 }
             } else {
+                // Apply the cursor velocity to the scroll area when the user releases the drag.
+                if content_response_option
+                    .as_ref()
+                    .is_some_and(|response| response.drag_stopped())
+                {
+                    for d in 0..2 {
+                        if scroll_enabled[d] {
+                            ui.input(|input| {
+                                state.vel[d] = input.pointer.velocity()[d];
+                            });
+                        } else {
+                            state.vel[d] = 0.0;
+                        }
+                    }
+                }
                 for d in 0..2 {
                     // Kinetic scrolling
                     let stop_speed = 20.0; // Pixels per second.