[REG] MacOs: Windows do not resize correctly in tabbed window mode

The described issue does not exist in Qt 5.12.1 and was partly introduced in Qt 5.12.2 and made more apparent in Qt 5.12.3.

To reproduce the issue, macOS Dock has to be configured to open all new windows in tabbed mode (which by default only happens in full screen mode):

Then, the attached small program demonstrates the issue.

Reproduction Steps:

1. Start the application
2. Click "new window" => new windows opens as a tab

The results for each Qt version mentioned above:

Qt 5.12.1 (all fine):

When the app enters tabbed mode, all windows are resized correctly.

Note that each window gets resize events for the required size adjust (320x175).

Qt 5.12.2 (wrong report size, scaled height)

 The windows are resized, but the old size is still reported. The widget looks stretched down to 320x175 in height.

No resize event for the tabbed mode change.

Qt 5.12.3 (cut off, not scaled)

The windows are no longer stretched in height, but cut off.

No resize event for the tabbed mode change.

General:

• in full screen mode it works
• removing the window shows the same issue
• resizing the window fixes the wrong size

I suspect it was somehow caused by 9f22ac0aa0254f20f9b26aec7b124d74141fdfcd but did not verify.