when you have enough tabs that they don't fit into a single row (this is in the normal tabs-on-top configuration),
AND the window is so narrow that it cannot even accomodate the width of 1 tab,
1) the tab outline for the 1st row is clipped, while the tab outline for subsequent rows' tabs are shortened to fit. (Inconsistent: all tabs should be drawn to fit and never get clipped).
2) the text labels are not clipped to the interior of their tabs, but overflow into the rest of the area and get clipped by it. Probably not calculating font metrics properly to size the text to fit under these circumstances.
reproducible by running SwingSet and making the window about 1 inch wide (the minimum).