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 242 matching lines @@
     ScreenshotDelegate* screenshot_delegate) {
   if (screenshot_delegate_) {
     DCHECK_EQ(screenshot_delegate_, screenshot_delegate);
     return;
   }
   screenshot_delegate_ = screenshot_delegate;
   mode_ = WINDOW;
 
   display::Screen::GetScreen()->AddObserver(this);
   for (aura::Window* root : Shell::GetAllRootWindows()) {
+    // Since we'll be pre-handling all mouse events, break any existing
+    // captures.
+    aura::client::GetCaptureClient(root)->SetCapture(nullptr);

[sky, 2016/10/05 15:57:35]

I'm mildly worried about side effects this may have and calling it part way
through Start. Also, could you use CancelMode instead? See DispatchCancelMode()
and make alt-tab respect that?

[Evan Stade, 2016/10/05 17:21:37]

I was actually thinking that any side effects would be desired. For example, I
just tested and found we have some kooky behavior if you trigger a screenshot
session while dragging tabs (tab is frozen in place and stays there after the
screenshot session is over). This fixes that by aborting the tab dragging. 

[sky, 2016/10/06 15:45:50]

Sorry for not being clear on side effects. I was more worried about side effects
in this class. I agree that canceling tab dragging is what you want to happen. 

I was hoping ET_CANCEL_MODE would trigger canceling drag and drop, but it
doesn't look like it does. So, it seems like canceling capture is the right
thing. My only request is ti do this before setting the delegate/mode.

[Evan Stade, 2016/10/06 16:32:34]

done (I assume you meant to move the whole loop?)

     layers_[root] = base::MakeUnique<ScreenshotLayer>(
         Shell::GetContainer(root, kShellWindowId_OverlayContainer)->layer(),
         true);
   }
   SetSelectedWindow(wm::GetActiveWindow());
 
   cursor_setter_.reset(new ScopedCursorSetter(
       Shell::GetInstance()->cursor_manager(), ui::kCursorCross));
 
   EnableMouseWarp(true);
 }
 
 void ScreenshotController::StartPartialScreenshotSession(
     ScreenshotDelegate* screenshot_delegate,
     bool draw_overlay_immediately) {
   // Already in a screenshot session.
   if (screenshot_delegate_) {
     DCHECK_EQ(screenshot_delegate_, screenshot_delegate);
     return;
   }
 
   screenshot_delegate_ = screenshot_delegate;
   mode_ = PARTIAL;
   display::Screen::GetScreen()->AddObserver(this);
   for (aura::Window* root : Shell::GetAllRootWindows()) {
+    // Since we'll be pre-handling all mouse events, break any existing
+    // captures.
+    aura::client::GetCaptureClient(root)->SetCapture(nullptr);
     layers_[root] = base::MakeUnique<ScreenshotLayer>(
         Shell::GetContainer(root, kShellWindowId_OverlayContainer)->layer(),
         draw_overlay_immediately);
   }
 
   if (!pen_events_only_) {
     cursor_setter_.reset(new ScopedCursorSetter(
         Shell::GetInstance()->cursor_manager(), ui::kCursorCross));
   }
 
@@ skipping 244 matching lines @@
 
 void ScreenshotController::OnDisplayMetricsChanged(
     const display::Display& display,
     uint32_t changed_metrics) {}
 
 void ScreenshotController::OnWindowDestroying(aura::Window* window) {
   SetSelectedWindow(nullptr);
 }
 
 }  // namespace ash