When vertically scrolling the iframe on this page: http://people.mozilla.org/~kgupta/tmp/iframe.html in the B2G browser, I see the following misbehaviours: 1) When the 1500-1600 pixel mark is reached, rendering stops below that point. The exact point at which rendering stops changes as you scroll, but it's within that range. This may be a displayport issue. 2) When further scrolling is attempted after the above point is reached, the iframe jumps to around the 1100-1200 pixel position. This is with today's m-c on an Unagi device.

The problem was that FrameMetrics::mCompositionBounds was computed incorrectly for subframes. Since bug 904533 also involved changes to how mCompositionBounds is computed, I combined my fix for this with the changes in that bug and uploaded the combined patch to that bug.