Currently when we put up the slow-script dialog I don't think we make any effort at all at not re-entering the page in the form of timers, network events and UI events. This breaks the JS run-to-completion model, and likely lots of pages (most that trigger the slow-script dialog?) For sync XHR we have code that attempts to disable various callbacks into the page while we spin the event loop for the XHR. We should try to leverage that code.
6 years ago