Tab close button -- intermittently fails to appear

Intermittently, the Tab close button does not appear. When this happens, the tab text is also incorrectly aligned. See the attachment.

This happens even though the tab-closing-policy is ALL_TABS and closeable is set to 'true' for every tab.

I can reproduce this with a program that allows the user to create a new tab by clicking on a button.... when I click several times in a row, the problem occurs once every 2nd or 3rd run of the program.