Add pointer capture API for stable drag handling

Add minimal pointer capture API to gpui::Window:
- capture_pointer(hitbox_id): starts capture for the given hitbox
- release_pointer(): releases capture
- captured_hitbox(): returns the captured hitbox, if any

When captured, HitboxId::is_hovered() returns true for the captured
hitbox regardless of actual hit testing. Capture is automatically
released on MouseUpEvent.

This enables drag operations (like scrollbar thumb dragging) to
continue working even when the pointer moves outside the element's
bounds during the drag.

crates/gpui/src/window.rs

@@ -506,6 +506,10 @@ impl HitboxId {
     ///
     /// See [`Hitbox::is_hovered`] for details.
     pub fn is_hovered(self, window: &Window) -> bool {
+        // If this hitbox has captured the pointer, it's always considered hovered
+        if window.captured_hitbox == Some(self) {
+            return true;
+        }
         let hit_test = &window.mouse_hit_test;
         for id in hit_test.ids.iter().take(hit_test.hover_hitbox_count) {
             if self == *id {
@@ -892,6 +896,9 @@ pub struct Window {
     pub(crate) pending_input_observers: SubscriberSet<(), AnyObserver>,
     prompt: Option<RenderablePromptHandle>,
     pub(crate) client_inset: Option<Pixels>,
+    /// The hitbox that has captured the pointer, if any.
+    /// While captured, mouse events route to this hitbox regardless of hit testing.
+    captured_hitbox: Option<HitboxId>,
     #[cfg(any(feature = "inspector", debug_assertions))]
     inspector: Option<Entity<Inspector>>,
 }
@@ -1316,6 +1323,7 @@ impl Window {
             prompt: None,
             client_inset: None,
             image_cache_stack: Vec::new(),
+            captured_hitbox: None,
             #[cfg(any(feature = "inspector", debug_assertions))]
             inspector: None,
         })
@@ -1999,6 +2007,26 @@ impl Window {
         self.mouse_position
     }
 
+    /// Captures the pointer for the given hitbox. While captured, all mouse move and mouse up
+    /// events will be routed to listeners that check this hitbox's `is_hovered` status,
+    /// regardless of actual hit testing. This enables drag operations that continue
+    /// even when the pointer moves outside the element's bounds.
+    ///
+    /// The capture is automatically released on mouse up.
+    pub fn capture_pointer(&mut self, hitbox_id: HitboxId) {
+        self.captured_hitbox = Some(hitbox_id);
+    }
+
+    /// Releases any active pointer capture.
+    pub fn release_pointer(&mut self) {
+        self.captured_hitbox = None;
+    }
+
+    /// Returns the hitbox that has captured the pointer, if any.
+    pub fn captured_hitbox(&self) -> Option<HitboxId> {
+        self.captured_hitbox
+    }
+
     /// The current state of the keyboard's modifiers
     pub fn modifiers(&self) -> Modifiers {
         self.modifiers
@@ -3890,6 +3918,11 @@ impl Window {
                 self.refresh();
             }
         }
+
+        // Auto-release pointer capture on mouse up
+        if event.is::<MouseUpEvent>() && self.captured_hitbox.is_some() {
+            self.captured_hitbox = None;
+        }
     }
 
     fn dispatch_key_event(&mut self, event: &dyn Any, cx: &mut App) {