Created attachment 694989 [details] Reduced test case I noticed this in Instantbird after turning on nglayout.debug.paint_flashing when I saw with the gecko profiler that more than 40% of the time spent while displaying a very large IM conversation is spent in -[NSView displayIfNeeded] The full profile is http://people.mozilla.com/~bgirard/cleopatra/#report=1b1d4e07a264986d3d1a4a2780cb9601c013b867 (and yes, I've got lots of other things to optimize in my JS code). Note: it's possible that this profile was taken with a message theme that used a background gradient on <body>. With the gradient the whole content area was repainted, without it only a part kept being repainted at the top, even though nothing had changed there. Here is a screencast of the surprising painting behavior: http://www.youtube.com/watch?v=QNn7qHUsyiA I've reduced the testcase (see attachment). In Firefox 17.0.1 (same gecko that Instantbird is currently using), I see a similar behavior: the first line is repainted all the time, but the repainted area has the length of the newly inserted line. When testing on a current nightly, the whole content area is redisplayed all the time. I tested only on OS X 10.8.
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