It was determined in https://bugs.webkit.org/show_bug.cgi?id=243401 that isMainThread() can not simply check if the current thread is the initial process thread but must compare the current thread against the thread set when initializeMainThreadPlatform is called to handle cases when WebKit is run under applications like Eclipse which don't initialize WebKit in the main process thread.
Pull request: https://github.com/WebKit/WebKit/pull/3497
Committed 253647@main (799dd04c3edb): <https://commits.webkit.org/253647@main>
Reviewed commits have been landed. Closing PR #3497 and removing active labels.