Created attachment 600649 [details]
testcase (clicking should have no visible effect)
At first, there are 4 red bars at the top right. After clicking, there are only 3.
I think this is different from bug 726460, but I'm not sure.
Created attachment 607805 [details] [diff] [review]
Created attachment 607807 [details] [diff] [review]
The testcase as reftest
(In reply to Jesse Ruderman from comment #1)
> I think this is different from bug 726460, but I'm not sure.
They're different sides of the same coin. This one is caused by the non-fluid continuation between two frames remaining non-fluid after their content has been deleted; bug 726460 is caused by the the fluid continuation between two frames remaining fluid after the content has been replaced by new content.