When a subresource is loaded during an event tracking loop (sliders and color wheel actions, for example), WebKit doesn't actually load the changes until the loop is ended. It appears that the data does get loaded during the loop, but the page isn't updated until the loop ends. Sample project and app displaying the bug: http://macrabbit.com/misc/webkit-event-tracking/WebKitEventTracking.zip
I have tracked the problem down to NSURLConnection. WebKit uses NSURLConnection in ResourceHandleMac.mm: start(Frame* frame). Since NSURLConnection refuses to send delegate methods during event tracking, all of WebKit inherits this behavior. Possible solutions: threading the resource load or fixing this bug in Foundation. The latter is probably more preferable.
(In reply to comment #1) > I have tracked the problem down to NSURLConnection. WebKit uses NSURLConnection > in ResourceHandleMac.mm: start(Frame* frame). Since NSURLConnection refuses to > send delegate methods during event tracking, all of WebKit inherits this > behavior. > > Possible solutions: threading the resource load or fixing this bug in > Foundation. The latter is probably more preferable. Foundation bugs/enhancement requests need to be reported through <https://bugreport.apple.com/>. If you do not have an ADC account, you may sign up for a free "online" account at <http://connect.apple.com/>. Please reference the Bugzilla bug there and provide the radar bug number here when you're done.
Related Radar bug: radar://5401429
To the Apple employees that work on WebKit: I am now aware of a fix for the NSURLConnection issue in Leopard, but I can't disclose it. You may want to contact John Montbriand at Developer Technical Support.