It is closely related to
https://bugreports.qt.io/browse/QTBUG-134249
that the issue is still "enter/leaveEvent being registered repeatedly like spamming". But it is on macOS only since it involves macos-only attributes.

Reproducer attached. There is no difference than the one I uploaded for QTBUG-134249, with only 2 lines added:

        setAttribute(Qt::WA_ShowWithoutActivating, true);
        setAttribute(Qt::WA_MacAlwaysShowToolWindow);

and 1 line modified:

topWidget->setWindowFlags(Qt::FramelessWindowHint | Qt::Tool | Qt::NoDropShadowWindowHint); //It is Qt::Tool only for QTBUG-134249

Run the reproducer as it is, then you can see the same enter/leaveEvent problem. The solution is to remove both Qt::FramelessWindowHint and Qt::NoDropShadowWindowHint, as well as Qt::WA_ShowWithoutActivating and Qt::WA_MacAlwaysShowToolWindow (basically comment out everything at line 15, 16 and 17). Those flags seem to mess up with mouse events.

Update: a one-line solution: use Qt::Popup instead of Qt::Tool. Given that Tool is Popup | Dialog, may something's off inside Dialog.