Fix partial screenshot/window screenshot when holding Alt+Tab.

ash/utility/screenshot_controller.cc

 // Copyright 2014 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 "ash/utility/screenshot_controller.h"
 
 #include <cmath>
 
 #include "ash/common/shell_window_ids.h"
 #include "ash/display/mouse_cursor_event_filter.h"
@@ skipping 281 matching lines @@
 
   if (!pen_events_only_) {
     cursor_setter_.reset(new ScopedCursorSetter(
         Shell::GetInstance()->cursor_manager(), ui::kCursorCross));
   }
 
   EnableMouseWarp(false);
 }
 
 void ScreenshotController::CancelScreenshotSession() {
+  for (aura::Window* root : Shell::GetAllRootWindows()) {
+    // Having pre-handled all mouse events, widgets that had mouse capture may
+    // now misbehave, so break any existing captures. Do this after the
+    // screenshot session is over so that it's still possible to screenshot
+    // things like menus.
+    aura::client::GetCaptureClient(root)->SetCapture(nullptr);
+  }
+
   mode_ = NONE;
   pen_events_only_ = false;
   root_window_ = nullptr;
   SetSelectedWindow(nullptr);
   screenshot_delegate_ = nullptr;
   display::Screen::GetScreen()->RemoveObserver(this);
   layers_.clear();
   cursor_setter_.reset();
   EnableMouseWarp(true);
 
@@ skipping 230 matching lines @@
 
 void ScreenshotController::OnDisplayMetricsChanged(
     const display::Display& display,
     uint32_t changed_metrics) {}
 
 void ScreenshotController::OnWindowDestroying(aura::Window* window) {
   SetSelectedWindow(nullptr);
 }
 
 }  // namespace ash