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..a9413f51738bcebf0777fda2d7f02bc1f1c296ce 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;
@@ -203,6 +206,10 @@ void EventDispatcher::ProcessKeyEvent(mojom::EventPtr event) {
 }
 
 void EventDispatcher::ProcessPointerEvent(mojom::EventPtr event) {
+  bool is_mouse_event =
+      event->pointer_data &&
+      event->pointer_data->kind == mojom::PointerKind::POINTER_KIND_MOUSE;
+
   const int32_t pointer_id = event->pointer_data->pointer_id;
   if (event->action == mojom::EVENT_TYPE_WHEEL ||
       (event->action == mojom::EVENT_TYPE_POINTER_MOVE &&
@@ -215,6 +222,8 @@ void EventDispatcher::ProcessPointerEvent(mojom::EventPtr event) {
       pointer_target.window =
           FindDeepestVisibleWindowForEvents(root_, surface_id_, &location);
     }
+    if (is_mouse_event && !mouse_button_down_)
+      mouse_cursor_source_window_ = pointer_target.window;
     DispatchToPointerTarget(pointer_target, event.Pass());
     return;
   }
@@ -230,6 +239,11 @@ void EventDispatcher::ProcessPointerEvent(mojom::EventPtr event) {
     if (!IsObservingWindow(target))
       target->AddObserver(this);
 
+    if (is_mouse_event) {
+      mouse_button_down_ = true;
+      mouse_cursor_source_window_ = target;
+    }
+
     pointer_targets_[pointer_id].window = target;
     pointer_targets_[pointer_id].in_nonclient_area =
         IsLocationInNonclientArea(target, location);
@@ -246,6 +260,18 @@ void EventDispatcher::ProcessPointerEvent(mojom::EventPtr event) {
       (event->pointer_data->kind != mojom::POINTER_KIND_MOUSE ||
        IsOnlyOneMouseButtonDown(event->flags));
 
+  if (should_reset_target && is_mouse_event) {
+    // 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. We have to set this
+    // before we perform dispatch because the Delegate is going to read this
+    // information from us.
+    mouse_button_down_ = false;
+    gfx::Point location(EventLocationToPoint(*event));
+    mouse_cursor_source_window_ =
+        FindDeepestVisibleWindowForEvents(root_, surface_id_, &location);
+  }
+
   DispatchToPointerTarget(pointer_targets_[pointer_id], event.Pass());
 
   if (should_reset_target) {
@@ -311,6 +337,9 @@ 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