If all of your monitors are running at 100%, we grandfather the FrontEnd into how things rendered in 12.0. You can opt-out of being grandfathered in by setting ScreenResolutionCompatibilityMode->False in the Options Inspector. If a display is running greater than 100%, then the FrontEnd runs in this new manner.

As for the clipped frame, I only see that when the global scale is set to 115%. 100% is fine. 125% is fine. You should email tech support and have them file a bug.

To be more inline with 12.0, the Magnification should be set to 0.75. i.e. 72/96. I wasn't able to immediately reproduce any clipping of the vertical frame, so it might be nice if you could supply an example. And probably the result of CurrentValue["WindowResolution"].