I've been hacking up an implementation of interruptible reflow. Laying out large pages is sometimes a source of unacceptable delays. It would be a better user experience if, during a long layout, we could detect that there's pending user input, get out of the layout code, handle the user input, and then resume the layout. In principle this isn't that hard now that we use dirty bits to track what needs to be laid out. It's simply a matter of making sure the right dirty bits are set as we bail out of reflow, and then making sure we enter reflow again eventually to complete the unfinished work. We also have to make sure we accurately distinguish "interruptible" reflows from "uninterruptible" reflows. For example, if a reflow is triggered by script asking for the geometry of some element, that's uninterruptible since returning with some work undone would give incorrect results and break things.
I have a patch that basically works, at least for pages that are dominated by block layout. But I've run into a severe problem: I don't know how to detect pending user input in Mac OS X or GTK :-(. On Windows, for about 20 years there's been a function called GetInputState which does exactly what we need. OS X and GTK/X11 just don't have anything like it. I've tried Appkit's nextEventMatchingMask; it sometimes processes some events, which is unacceptable. X11 doesn't seem to provide a way to peek the event queue without blocking; the only nonblocking event queue reading APIs always remove the event from the queue if they find one.
OS X and GTK suck, Windows rules. Prove me wrong, fanboys!