Title not displayed for dialog boxes under MacOS

[elmm]

tiled.alert, confirm don't show title under MacOS.

Platform: macOS Catalina 10.15.7
Editor: Version 1.4.2-50-g76796ffb (was the same behaviour with previous version, based on QT 5.12)

[bjorn]

This turned out to be pretty much a Qt feature, which hides the message box titles on macOS for conformance to Apple's HIG. However, in this case I think could be more helpful to show the title, so I'm applying the workaround.