Do not release capture when starting a move loop on Desktop Linux

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h"
 
 #include <X11/extensions/shape.h>
 #include <X11/extensions/XInput2.h>
 #include <X11/Xatom.h>
 #include <X11/Xregion.h>
@@ skipping 903 matching lines @@
     OnHostResized(bounds.size());
     ResetWindowRegion();
   }
 }
 
 gfx::Point DesktopWindowTreeHostX11::GetLocationOnNativeScreen() const {
   return bounds_.origin();
 }
 
 void DesktopWindowTreeHostX11::SetCapture() {
+  if (HasCapture())
+    return;
+
   // Grabbing the mouse is asynchronous. However, we synchronously start
   // forwarding all mouse events received by Chrome to the
   // aura::WindowEventDispatcher which has capture. This makes capture
   // synchronous for all intents and purposes if either:
   // - |g_current_capture|'s X window has capture.
   // OR
   // - The topmost window underneath the mouse is managed by Chrome.
-  if (g_current_capture)
-    g_current_capture->OnHostLostWindowCapture();
+  DesktopWindowTreeHostX11* old_capturer = g_current_capture;
   g_current_capture = this;
+  if (old_capturer)
+    old_capturer->OnHostLostWindowCapture();
 
   unsigned int event_mask = PointerMotionMask | ButtonReleaseMask |
                             ButtonPressMask;
   XGrabPointer(xdisplay_, xwindow_, True, event_mask, GrabModeAsync,
                GrabModeAsync, None, None, CurrentTime);
 }
 
 void DesktopWindowTreeHostX11::ReleaseCapture() {
   if (g_current_capture == this) {
     // Release mouse grab asynchronously. A window managed by Chrome is likely
@@ skipping 959 matching lines @@
   if (linux_ui) {
     ui::NativeTheme* native_theme = linux_ui->GetNativeTheme(window);
     if (native_theme)
       return native_theme;
   }
 
   return ui::NativeTheme::instance();
 }
 
 }  // namespace views