As seen on mediawiki.org (running 1.23wmf2 (020fe63)) in Firefox 24.

When editing in VisualEditor, the first time I click on the menu icon, the dropdown is slightly too small and a horizontal scroll bar appears inside the dropdown.

The next time I click on the icon, the dropdown's width can accommodate the text and there is no scrollbar in the dropdown itself, but a horizontal scrollbar appears at the page level to accommodate the dropdown's extra width.

Then, rinse and repeat: every other time, there's a scroll bar in the dropdown, and the next one there is one at the bottom of the page.

Version: unspecified
Severity: normal