Before, we were calling DestroyWindow on the browser panel HWND from the
CEF UI thread, which meant that the main program thread had to process
window messages in order for that message to successfully through as
DestroyWindow blocks on the WM_DESTROY/WM_NCDESTROY window messages.
Unfortunately, this also had the side-effect of processing all queued Qt
events, which, when called in the destructor of a window like this, can
result in undefined behavior; specifically crashes, which were occurring
due to this. Especially when browser panels were in docks and docked to
the main window.
So, instead of calling DestroyWindow in another thread and then
processing events in the main thread, call DestroyWindow directly in the
main thread to get WM_DESTROY/WM_NCDESTROY and only those processed on
the spot. This appears to fix the crashes due to the undefined
behavior when closing browser panels.
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