Re-land reverted r269593; it was a red herring for ash_unittests

Avoids releasing capture in EndMoveLoop if capture was switched to another window while dragging.
This variant relies on checking if a drag input window still has capture before releasing the grab.
A better alternative is in CL https://codereview.chromium.org/262893002/.

TBR=sadrul@chromium.org
BUG=363503
TEST=Follow the exact steps in the bug description.
TEST=interactive_ui_tests --gtest_filter=*TabDragging*

Review URL: https://codereview.chromium.org/272363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269688 0039d316-1c4b-4281-b951-d872f2087c98

ui/views/widget/desktop_aura/x11_whole_screen_move_loop.cc

@@ -62,6 +62,7 @@ X11WholeScreenMoveLoop::X11WholeScreenMoveLoop(
       should_reset_mouse_flags_(false),
       grab_input_window_(None),
       canceled_(false),
+      has_grab_(false),
       weak_factory_(this) {
   last_xmotion_.type = LASTEvent;
 }
@@ -136,6 +137,11 @@ uint32_t X11WholeScreenMoveLoop::DispatchEvent(const ui::PlatformEvent& event) {
       }
       break;
     }
+    case FocusOut: {
+      if (xev->xfocus.mode != NotifyGrab)
+        has_grab_ = false;
+      break;
+    }
     case GenericEvent: {
       ui::EventType type = ui::EventTypeFromNative(xev);
       switch (type) {
@@ -253,8 +259,11 @@ void X11WholeScreenMoveLoop::EndMoveLoop() {
 
   // Ungrab before we let go of the window.
   XDisplay* display = gfx::GetXDisplay();
-  XUngrabPointer(display, CurrentTime);
-  XUngrabKeyboard(display, CurrentTime);
+  // Only ungrab pointer if capture was not switched to another window.
+  if (has_grab_) {
+    XUngrabPointer(display, CurrentTime);
+    XUngrabKeyboard(display, CurrentTime);
+  }
 
   // Restore the previous dispatcher.
   nested_dispatcher_.reset();
@@ -295,6 +304,7 @@ bool X11WholeScreenMoveLoop::GrabPointerAndKeyboard(gfx::NativeCursor cursor) {
     DLOG(ERROR) << "Grabbing pointer for dragging failed: "
                 << ui::GetX11ErrorString(display, ret);
   } else {
+    has_grab_ = true;
     XUngrabKeyboard(display, CurrentTime);
     ret = XGrabKeyboard(
         display,

ui/views/widget/desktop_aura/x11_whole_screen_move_loop.h

@@ -91,6 +91,9 @@ class X11WholeScreenMoveLoop : public ui::PlatformEventDispatcher {
   // pressing escape).
   bool canceled_;
 
+  // Keeps track of whether we still have a pointer grab at the end of the loop.
+  bool has_grab_;
+
   // A Widget is created during the drag if there is an image available to be
   // used during the drag.
   scoped_ptr<Widget> drag_widget_;