Are you encountering this issue in specific browsers, or is it consistent across all browsers? Also, can you take a look at the fiddle I've included below? It's a simple tab panel with a grid.
If you don't see the issue in the fiddle, it may be an issue with your configuration. Can you either share that configuration here, or (ideally) fork and modify the fiddle I've included in order to replicate what you're seeing? I'll be happy to test it along with you.
Finally, have you made any changes to the theme or added any custom stylesheets that might be adding/overriding the styles?