The only thing a broad google search came up with was this post:
Quote:
Re: Two (Stacked) Title Bars in OSX
Quote
Post by Klaus » Fri Nov 09, 2018 11:49 pm
Hi Andy,
this is a macOS setting:
(translated from a german system, so it might read a bit different on your machine)
System Settings -> Dock -> Prefer tabs when opening documents
I set it to -> Manually
after i also experienced this scary thing effect. 
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Do you have this system setting set by chance? Are you somehow tabbing multiple windows of the same app?
If so, I really do not think that is a good idea for Qt based apps.