The mock scrollbar Setting is set after view creation in DRT. If any scrollbars are created during view creation and do not get replaced later, they will be platform scrollbars instead of mock scrollbars (because setting the setting doesn't force the scrollbars to be recreated). WebKitTestRunner gets around this by setting the preference before creating the view; doing so in DRT as well makes everything happy.
Created attachment 230013 [details]
At some point we should probably fix the FIXME in Settings::setMockScrollbarsEnabled, but that is harder.
Should we remove that from resetWebPreferencesToConsistentValues()? Can tests change the value of this preference at all?
(In reply to comment #3)
> Should we remove that from resetWebPreferencesToConsistentValues()? Can tests change the value of this preference at all?
Since it's a WebCore setting and exposed via internals, they CAN, but shouldn't, and the only tests that set it set it to true (why‽).
Filed bug 132085 to kill it.