Reset Cursor type when pointer exits

If user places the pointer/mouse on any end of a browser window, cursor
type changes accordingly, to indicated a possible interactive resize operation.

If the user crosses the browser window bounds through the same end, the mouse
type stored in ui::CursorManager isn't reset. If pointer reenters the
window through the same end, Aura tries to set the cursor type to the
same as previously (to indicate it is over the same end). During the
call chain, CursorManager::SetCursor compares the last Cursor stored
and the new one and updates the cursor instance if they differ.

In case of regular Chrome/x11 builds, the check passes because
CursorData is set to the Cursor instance, and values always differ
from the CursorData cached by CursorManager.
In case of Linux/Ozone builds, CursorData is only set on Ozone side,
so the call bail out and cursor is not updated.

Patch fixes it by resetting the cursor type whenever pointer leaves
the browser window. This ensures, when reentering from the same end, the
comparison in CursorManager::SetCursor passes, and cursor gets updated.

Issue Igalia#317

ui/wm/core/compound_event_filter.cc

@@ -97,6 +97,18 @@ void CompoundEventFilter::UpdateCursor(aura::Window* target,
   aura::client::CursorClient* cursor_client =
       aura::client::GetCursorClient(root_window);
   if (cursor_client) {
+    // In some configurations, notably Linux/Ozone, cursor data are only
+    // set in Ozone side. This makes the check in CursorManager::SetCursor
+    // unreliable and fail in some circumstances where it passes on regular
+    // Linux/X11 builds.
+    // To make sure, cursor is properly set when it reenters a browser window
+    // through the same end, we reset it when it exits, otherwise the check in
+    // CursorManager::SetCursor will comparent Cursors of the same type.
+    if (event->type() == ui::ET_MOUSE_EXITED) {
+      cursor_client->SetCursor(ui::CursorType::kPointer);
+      return;
+    }
+
     gfx::NativeCursor cursor = target->GetCursor(event->location());
     if ((event->flags() & ui::EF_IS_NON_CLIENT)) {
       if (target->delegate()) {
@@ -196,11 +208,14 @@ void CompoundEventFilter::OnMouseEvent(ui::MouseEvent* event) {
   // on Desktop for testing, or a bug in pointer barrier).
   if (!(event->flags() & ui::EF_FROM_TOUCH) &&
        (event->type() == ui::ET_MOUSE_ENTERED ||
+        event->type() == ui::ET_MOUSE_EXITED ||
         event->type() == ui::ET_MOUSE_MOVED ||
         event->type() == ui::ET_MOUSE_PRESSED ||
         event->type() == ui::ET_MOUSEWHEEL)) {
-    SetMouseEventsEnableStateOnEvent(window, event, true);
-    SetCursorVisibilityOnEvent(window, event, true);
+    if (event->type() != ui::ET_MOUSE_EXITED) {
+      SetMouseEventsEnableStateOnEvent(window, event, true);
+      SetCursorVisibilityOnEvent(window, event, true);
+    }
     UpdateCursor(window, event);
   }