[Reg 5.15.12 -> 6.4.2] QProxyStyle returns a rect with negative width

In the example a QTabBar is used in a special way: The last tab is "fixed", e.g. it should always be visible, even when the scroll buttons are shown. To do this, a corner widget is used and implemented a widget that can draw a single tab with a QStylePainter (CornerTab.h).

In Qt 5.15.12 this works fine, but with Qt6 only the panel is drawn and not the text.
The debug output of the proxy style shows that the text is tried to draw with a rect with a negative width. The rect for the tab panel is correct, but the rect for the tab text is calculated with negative witdth in Qt6.

To reproduce the issue, please run the attached example. With Qt6, the text is not drawn to to rightmost tab and the debug output shows the negative value for the rect width.