I am getting the same issue, with MediaWiki 1.30.0 and PHP 7.0.27, with the same version of Tabs (but from the master branch). From what I can see, the error is due to the styles not being applied and it appears to be a problem with the extension, as I get in the Debug Log when I turn on the Debug Toolbar of MediaWiki:
[resourceloader] Unexpected general module "ext.tabs" in styles queue.
I've been looking to see how this can be fixed, but cannot find a fix for it. Things I tried:
- Commenting out usages of addModuleStyles() and addModuleScripts() in Tabs.body.php
- Changing extension.json (but this problem was occurring even with the version of Tabs from the REL1_30 branch and editing Tabs.php instead) to have the styles and scripts separate, which removes the error but doesn't load the styles
- Changing the uses of addModuleStyles() and addModuleScripts() to just addModules()
None of these have worked, though.