Split out from bug 646359. After some heavy debugging I realized that the reason why the original patch in attachment 495873 [details] [diff] [review] had caused bug 654419 was not what I said in bug 646359 comment 1, but that when encountering line breaks in preformatted text in nsBidiPresUtils::TraverseFrames we can create continuations, and in that case we need to dirty the line. This is the last hunk of the patch. On the other hand, I think we were dirtying lines too often in ResolveParagraph (which covered up the fact that we weren't doing it when we should), so the rest of the patch moves the dirtying to points where we are changing or creating continuations. However! the try run https://tbpl.mozilla.org/?tree=Try&rev=4089c3ced4a8 shows that this breaks layout/reftests/text-overflow/xulscroll.html, but that seems to be because of a separate bug, which again was beeing hidden by the unnecessary dirtying.

I hijacked bug 936936 to be about the -moz-box incremental reflow problem. (I assume you'll tweak xulscroll.html in this bug to avoid that bug for now.)

Added workaround for bug 936935 to xulscroll.html as Mats suggested.