(The nsLineBox::SlideBy version of bug 1089388.) This method takes a parameter that's defined as a block-dir coordinate delta, but then applies it as a physical y-coordinate delta. As a result, we get messed-up overflow areas for lines within blocks that have vertical writing mode. I don't have a simple standalone testcase for this, but it definitely leads to flaky painting behavior (as one would expect), and fixing it provides a much more stable experience.

Comment on attachment 8511918 [details] [diff] [review] Convert nsLineBox::SlideBy to use logical coordinates. Review of attachment 8511918 [details] [diff] [review]: ----------------------------------------------------------------- ::: layout/generic/nsLineBox.h @@ +473,5 @@ > mContainerWidth = aContainerWidth; > mBounds.BStart(mWritingMode) += aDBCoord; > if (mData) { > + nsPoint physicalDelta = mozilla::LogicalPoint(mWritingMode, 0, aDBCoord). > + GetPhysicalPoint(mWritingMode, 0); It took me a little time to convince myself that it's kosher to overload nsPoint and nsLogicalPoint like this for offset coordinates with zero container-width, but it gives the right results so I'm not complaining.