How to reproduce:
Option 1:

1. Open 1 view
2. Open a 2nd view. The tab button of the first view will be overpainted by the 2nd view. At the place where the 2nd button should be there is a littel rendering artefact.
3. For 3rd view and more everything works fine.

Option 2:

1. Open 1 view
2. Open a 2nd view and move it to its own render part.
3. Open a 3rd view and move it between the two render parts

->as soon as you drop it into the new render part the bug is visible.

If render parts are minimized to an extend that the rerendering of the button row is needed, the tab buttons are also rendered correctly.