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

ash/utility/screenshot_controller.cc

Index: ash/utility/screenshot_controller.cc
diff --git a/ash/utility/screenshot_controller.cc b/ash/utility/screenshot_controller.cc
index 64e4f99785f09fd729063fb9d05434708c248969..af86bb06e52bf5190f41e4bf9e124cefa600bd82 100644
--- a/ash/utility/screenshot_controller.cc
+++ b/ash/utility/screenshot_controller.cc
@@ -260,6 +260,9 @@ void ScreenshotController::StartWindowScreenshotSession(
 
   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);
@@ -285,6 +288,9 @@ void ScreenshotController::StartPartialScreenshotSession(
   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);