mus: Let clients set the cursor of their window.

components/mus/ws/event_dispatcher.cc

Index: components/mus/ws/event_dispatcher.cc
diff --git a/components/mus/ws/event_dispatcher.cc b/components/mus/ws/event_dispatcher.cc
index 384f9ba81f95b1c2d41d364a79a8a2653dabbe13..e55f6f94ec982e83a4f7cfb042a0cf3da9486275 100644
--- a/components/mus/ws/event_dispatcher.cc
+++ b/components/mus/ws/event_dispatcher.cc
@@ -139,7 +139,10 @@ class EventMatcher {
 ////////////////////////////////////////////////////////////////////////////////
 
 EventDispatcher::EventDispatcher(EventDispatcherDelegate* delegate)
-    : delegate_(delegate), root_(nullptr) {}
+    : delegate_(delegate),
+      root_(nullptr),
+      mouse_button_down_(false),
+      mouse_cursor_source_window_(nullptr) {}
 
 EventDispatcher::~EventDispatcher() {
   std::set<ServerWindow*> pointer_targets;
@@ -261,6 +264,28 @@ void EventDispatcher::DispatchToPointerTarget(const PointerTarget& target,
   if (!target.window)
     return;
 
+  if (event->pointer_data->kind == mojom::PointerKind::POINTER_KIND_MOUSE) {
+    if (event->action == mojom::EventType::EVENT_TYPE_POINTER_MOVE) {
+      // Only change the last targeted window when we are moving without
+      // dragging.
+      if (!mouse_button_down_)
+        mouse_cursor_source_window_ = target.window;
+    } else if (event->action == mojom::EventType::EVENT_TYPE_POINTER_UP ||
+               event->action == mojom::EventType::EVENT_TYPE_POINTER_CANCEL) {
+      mouse_button_down_ = false;
+
+      // When we release the mouse button, we want the cursor to be sourced
+      // from the window under the mouse pointer, even though we're sending the
+      // button up event to the window that had implicit capture.
+      gfx::Point location(EventLocationToPoint(*event));
+      mouse_cursor_source_window_ =
+          FindDeepestVisibleWindowForEvents(root_, surface_id_, &location);
+    } else if (event->action == mojom::EventType::EVENT_TYPE_POINTER_DOWN) {
+      mouse_button_down_ = true;
+      mouse_cursor_source_window_ = target.window;
+    }
+  }
+
   gfx::Point location(EventLocationToPoint(*event));
   gfx::Transform transform(GetTransformToWindow(surface_id_, target.window));
   transform.TransformPoint(&location);
@@ -311,6 +336,9 @@ void EventDispatcher::OnWindowVisibilityChanged(ServerWindow* window) {
 
 void EventDispatcher::OnWindowDestroyed(ServerWindow* window) {
   CancelPointerEventsToTarget(window);
+
+  if (mouse_cursor_source_window_ == window)
+    mouse_cursor_source_window_ = nullptr;
 }
 
 }  // namespace ws