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..f9eadb7b68767b72a54b2a787122a7be88934af3 100644
--- a/ash/utility/screenshot_controller.cc
+++ b/ash/utility/screenshot_controller.cc
@@ -299,6 +299,14 @@ void ScreenshotController::StartPartialScreenshotSession(
 }
 
 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;