The problem is that non accelerated compositing forceRepaint implementation is doing a layout and then calling display. The layout makes the drawing area enter in AC mode and display asserts that we have a layer tree host. forceRepaint shouldn't do the layout because display already does that and it correctly handles the case of entering AC mode during the layout. It shouldn't call setNeedsDisplay either, because that schedules a display, but we are going to display synchronously.
Created attachment 300016 [details]
Committed r211346: <http://trac.webkit.org/changeset/211346>