The workarounds were made because of conflicts with running multiple UI
threads at once on macOS, which macOS can't do very well, and would be
susceptible to crashes. This would cause crashes not only on startup
but seemingly at random when using the browser source on macOS. The
original "fix" was a hack to try to minimize UI code and browser UI code
from executing at the same time. The macOS initial scene loading was
deferred until all Qt-related and main window initialization was
completed. Although this worked to some extent to prevent conflicts, it
made it so that there was an initial period on startup where the entire
UI seemed "blank" for users, and it was still possible for the main UI
thread and the browser UI thread to clash, causing crashes seemingly at
random for users.
The external message pump method of CEF is the solution to the problem,
which is the method which allows the main UI thread to share events with
CEF. To do this, all CEF operations need to be performed in the UI
thread (Qt's main thread), and CefDoMessageLoopWork() needs to be called
when CefApp::OnScheduleMessagePumpWork callback is triggered. A number
of other issues had to be solved as well, such as CefBrowser references
getting "stuck" in the Qt event queue.
With this, macOS no longer needs to do the "deferred load" hack, and
browsers are now much more stable and no longer as susceptible to
seemingly random crashes, improving overall program stability when
browsers are used.
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