See attached testcase. The red-bordered test <div> should remain in the same place within the black-bordered <div> that contains it; but when writing-mode is set to vertical-rl, it disappears far off the bottom of the screen. (Scroll all the way down to find it, with garbled border as well as misplaced.)

AFAICT, this is caused by writing-mode mixup in nsBlockReflowContext::ComputeCollapsedBStartMargin, resulting in a margin of NS_UNCONSTRAINEDSIZE (or thereabouts) above the orthogonal <div> in this example. I've got an experimental patch that I'll post for review if tryserver is happy with it.

Here's a reftest for the issue here, checking that the writing-mode of an inner div doesn't affect its placement. It'll pass harmlessly when writing-mode support isn't enabled.

Minor update to fix a failure in conjunction with bug 1083892's patch and test.

https://hg.mozilla.org/integration/mozilla-inbound/rev/8f0b367ed696 https://hg.mozilla.org/integration/mozilla-inbound/rev/c0b74388316a

https://hg.mozilla.org/mozilla-central/rev/8f0b367ed696 https://hg.mozilla.org/mozilla-central/rev/c0b74388316a