This change provides a generic solution for active (key) window tracking that
works with both Views-hosted and native windows on MacOS. With this new approach
we can now successfully route top menu actions to the currently active window.
Prior to this change CEF's Views API was using focus notifications as a proxy
for window activation notifications. That doesn't work on MacOS where NSWindow
activation (key status) is independent of NSView focus (first responder) status,
and changes in activation don't necessarily generate focus notifications (see
NativeWidgetMac::OnWindowKeyStatusChanged). To make this work reliably on all
platforms we now expose a CefWindowDelegate::OnWindowActivationChanged callback.
This change also fixes an uninitialized variable
(RootWindowMacImpl::with_extension_) that was causing flaky behavior in
RootWindowManager::OnRootWindowActivated.
To test:
1. Run `cefclient [--use-views]`
2. Select Popup Window from the Tests menu. Do not explicitly activate the popup
window (e.g. don't click on it).
3. Verify that further Tests menu actions go to the popup window.
4. Change activation to a first window by clicking on it. Verify that Test
menu actions go to that window.
5. Close the currently active window. Do not explicitly activate the remaining
window (e.g. don't click on it).
6. Verify that Test menu actions go to the only remaining window.
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