I have seen usual things happen with object like this before, I can't tell what cause the problem, but usually it can be corrected by remove the style. I've seen this happen with converted databases from 11 to 12 themes.
The icon with the slash at the top of the inspector under style custom will remove all setting. Click the next to last icon to reapply the default setting. Then you will have to reapply your other custom setting.
I can't reproduce this on one of my files. What theme are you using? I tried with both a classic themed layout from a converted layout and also on a layout with the Cool Grey theme.
If S Chamblee's suggestion doesn't work, you might also test this on a new layout in the file to see if you get the same results.
Well, removing the style didn't help, and I'm not using themes. So I bit the bullet and rebuilt the tab control from scratch. All is fine now, but I guess I'll never know what the problem was. Thanks for the suggestions.
and I'm not using themes.
In FileMaker 12, you are always using a theme. If you do not pick one, one is chosen for you.
Not sure which one it is then. "Change Theme" comes up with "Classic" hilited, but if I hit OK, it radically changes my layout.
Clicking OK re-applies the default styles for the selected theme.
So it would appear that you are using the classic theme, but where you have extensively modified the original styles.
Not really a major point at this juncture, but if we had been able to select a specific theme and then gotten this result, we might have had a bug to report to FileMaker.