When zooming in a window that has been zoomed out, sometimes a band of content across the top of the window is not refreshed, so the old differently-scaled content remains visible. I can reproduce this on current Nightly (FF17) and Aurora (16), but not on Beta (15) or release versions. STR (shows the issue reliably for me on MacBook Pro): * Load a bugzilla page such as this one. * Press Cmd-0 (command-zero) to ensure the page is at default zoom level. * Press Cmd-- (command-minus) five times to zoom out completely. * Press Cmd-= (command-equals) once to zoom back in by one step. At this point, I see a strip of unrefreshed content across the top of the window, as in the attached screenshot. Alternative STR, equally reliable for me: * Load about:home and reset zoom to default. * Zoom out (Cmd-minus) four times, then in (Cmd-equals) twice. Note the residue of the zoomed-out word "mozilla" at top right of the window. This reproduces both with and without hardware acceleration enabled. (Filing this in Graphics, but I suppose it could also be Widget or Layout-related.)

After my Aurora and Nightly builds updated themselves, I can no longer reproduce this. So apparently it's been fixed in the last few days - closing, sorry for the spam!