Reduce the fullscreen window height by 1px on activation loss.

ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
index f79b3c42baf5477962e15f31e634c799c5c5eb2d..8fb9819f219fa7980136985d97712cdab80c839e 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
@@ -696,7 +696,9 @@ void DesktopWindowTreeHostWin::HandleAppDeactivated() {
   native_widget_delegate_->EnableInactiveRendering();
 }
 
-void DesktopWindowTreeHostWin::HandleActivationChanged(bool active) {
+void DesktopWindowTreeHostWin::HandleActivationChanged(
+    bool active,
+    HWND window_gaining_or_losing_activation) {
   // This can be invoked from HWNDMessageHandler::Init(), at which point we're
   // not in a good state and need to ignore it.
   // TODO(beng): Do we need this still now the host owns the dispatcher?
@@ -706,6 +708,26 @@ void DesktopWindowTreeHostWin::HandleActivationChanged(bool active) {
   if (active)
     OnHostActivated();
   desktop_native_widget_aura_->HandleActivationChanged(active);
+
+  if (!::IsWindow(window_gaining_or_losing_activation))
+    window_gaining_or_losing_activation = ::GetForegroundWindow();
+
+  // If the window losing activation is a fullscreen window and it is on the

[sky, 2016/02/18 17:18:47]

Is there a reason not to do this any time the active window is fullscreen and
loses activation? Why only do some of the time?

[ananta, 2016/02/18 20:29:28]

We only want to do this if we are losing activation to a window on the same
monitor. Across monitors this works correctly. Is that what you are asking?

[sky, 2016/02/18 22:12:40]

I'm asking why not do this:

if (!active && IsFullscreen())
  SetFullscreen(false);

Even if it works now cross monitors, why not do it always?
I'm also asking if we gain focus and were fullscreen should set
SetFullscreen(true)?

+  // same monitor as the window being activated, then we switch away from full
+  // screen mode. This is to work around an apparent bug in the Windows taskbar
+  // where in it tracks fullscreen state on a per thread basis. This causes it
+  // not be a topmost window when any window on a thread which has a fullscreen
+  // window is active. Thus affecting the way these windows interact with the
+  // taskbar, i.e by obscuring it, causing autohide to not work correctly, etc.
+  // This works correctly across monitors.
+  if (!active && IsFullscreen() &&
+      ::IsWindow(window_gaining_or_losing_activation)) {
+    if (MonitorFromWindow(window_gaining_or_losing_activation,
+        MONITOR_DEFAULTTONEAREST) ==
+        MonitorFromWindow(GetHWND(), MONITOR_DEFAULTTONEAREST)) {
+      SetFullscreen(false);
+    }
+  }
 }
 
 bool DesktopWindowTreeHostWin::HandleAppCommand(short command) {