Set capture to the window being dragged when dragging a window on Ash

ash/wm/toplevel_window_event_handler.cc

Index: ash/wm/toplevel_window_event_handler.cc
diff --git a/ash/wm/toplevel_window_event_handler.cc b/ash/wm/toplevel_window_event_handler.cc
index 610261f9f78b21a0c2fce52c4091a2e9ace5232d..cc0362e0ece038b331aca7a1aba5289937ffb18d 100644
--- a/ash/wm/toplevel_window_event_handler.cc
+++ b/ash/wm/toplevel_window_event_handler.cc
@@ -110,6 +110,9 @@ class ToplevelWindowEventHandler::ScopedWindowResizer
   ToplevelWindowEventHandler* handler_;
   scoped_ptr<WindowResizer> resizer_;
 
+  // Whether ScopedWindowResizer grabbed capture.
+  bool grabbed_capture_;
+
   DISALLOW_COPY_AND_ASSIGN(ScopedWindowResizer);
 };
 
@@ -117,14 +120,24 @@ ToplevelWindowEventHandler::ScopedWindowResizer::ScopedWindowResizer(
     ToplevelWindowEventHandler* handler,
     WindowResizer* resizer)
     : handler_(handler),
-      resizer_(resizer) {
-  resizer_->GetTarget()->AddObserver(this);
-  wm::GetWindowState(resizer_->GetTarget())->AddObserver(this);
+      resizer_(resizer),
+      grabbed_capture_(false) {
+  aura::Window* target = resizer_->GetTarget();
+  target->AddObserver(this);
+  wm::GetWindowState(target)->AddObserver(this);
+
+  if (!target->HasCapture()) {

[flackr, 2014/12/16 21:28:21]

If this is to accomodate X11 already having capture, can we use #if
!defined(USE_X11) instead?

Alternately we could just capture again on X11 assuming it will be a noop.

[pkotwicz, 2014/12/17 04:48:12]

The reason that I do this is because I want to enforce:
"If ScopedWindowResizer grabbed capture, it should release capture."
I think that things are more straightforward this way.

In particular, I want what happens with capture in this scenario to be clear:
- Click on tab
- Drag tab
- Press escape

[flackr, 2014/12/17 19:39:15]

Acknowledged.

+    grabbed_capture_ = true;
+    target->SetCapture();

[flackr, 2014/12/16 21:28:21]

Maybe we should also pay attention to capture loss and stop the drag in such an
event?

[pkotwicz, 2014/12/17 04:48:12]

This would be bad. The tab dragging code switches which aura::Window has capture
prior to ending the drag. See TabDragController::DragBrowserToNewTabStrip()

[pkotwicz, 2014/12/17 05:18:36]

Actually, we already pay attention to loss of capture.
This is why the tab drag is aborted when you use accelerator keys to take a
partial screenshot.

[flackr, 2014/12/17 19:39:15]

Acknowledged, my main concern was losing capture to another application in
windows (or as you mentioned the partial screenshot shortcut). As long as it's
handled looks good.

+  }
 }
 
 ToplevelWindowEventHandler::ScopedWindowResizer::~ScopedWindowResizer() {
-  resizer_->GetTarget()->RemoveObserver(this);
-  wm::GetWindowState(resizer_->GetTarget())->RemoveObserver(this);
+  aura::Window* target = resizer_->GetTarget();
+  target->RemoveObserver(this);
+  wm::GetWindowState(target)->RemoveObserver(this);
+  if (grabbed_capture_)
+    target->ReleaseCapture();
 }
 
 bool ToplevelWindowEventHandler::ScopedWindowResizer::IsMove() const {