A div with an absolute position that is pinned to the bottom will not update as the window resizes only vertically. Any horizontal resize will make the block adjust it's size/position. The block will only resize once the border touches the frame edge.
Created attachment 4786 [details] Test Case resize this test case vertically
*** Bug 7081 has been marked as a duplicate of this bug. ***
This bug is similar to bug 6881. The fix there was to assure that positioned blocks are repositioned if their container's height changed, since that's what affects their position. The rendering root is a special case since it uses its availableHeight (essentially the viewport's height) for positioning instead of height. However, layoutBlock can't check for changes to availableHeight, since it's already changed by the time it's called. I'm going to propose a patch that always repositions if isRoot().
Created attachment 6335 [details] Always reposition the root's positioned children
Comment on attachment 6335 [details] Always reposition the root's positioned children You missed flexbox again. Need to patch it and then also include it in the test case.
I guess you need 2 test cases, since you have to use the root element to test. :)
Created attachment 6341 [details] Always reposition the root's positioned children Oops. Applied to flexbox and added it to the test.
Comment on attachment 6341 [details] Always reposition the root's positioned children r=me
Landed in r12712.