Breeze style problem since Qt 6.8.2

[Tobias Leupold]

E-Mail von Tobias Leupold vom Montag, 17. März 2025, 22:50:
> E-Mail von Albert Astals Cid vom Montag, 17. März 2025, 20:38:
> > Can you bisect in which commit the behaviour was introduced?
> 
> According to what was said on the Qt forums, it should be those two:
> 
> https://codereview.qt-project.org/c/qt/qtbase/+/601315
> https://codereview.qt-project.org/c/qt/qtbase/+/613831
> 
> cf. https://forum.qt.io/topic/160958/

... and as another follow-up:

I messed around with this stuff a bit now.

Using Qt 6.8.1, when no style sheet is set, sizeFromContents() is requested 
from Breeze when a QSpinBox should be drawn. It returns QSize(52, 32), and the 
QSpinBox's height is rendered as 32 px, most probably by the default Qt style 
class, I didn't find out yet where this happens.

If a style sheet is set, QStyleSheetStyle::sizeFromContents() comes into play. 
But Breeze's sizeFromContents() is not called in this case. The size stays 
initialized to

    QRenderRule rule = renderRule(w, opt);
    QSize sz = rule.adjustSize(csz);

which is QSize(36, 20).

Later on, (rule.hasBox() || rule.hasBorder() || !rule.hasNativeBorder()) 
matches, and the size is increased to QSize(48, 32) by sz = rule.boxSize(sz). 
The resulting QSpinBox is thus also drawn with a height of 32 px (looks like 
coincidence to me, as Breeze isn't involved at all?!).


This changed in Qt 6.8.2:

When no style sheet is set, the behavior is the same. Breeze's 
sizeFromContents() is called, returns QSize(52, 32), and the QSpinBox's height 
is rendered as 32 px.

But as soon as a style sheet is set, QStyleSheetStyle::sizeFromContents() 
takes over. But it behaves differently now: The initial size is overwritten 
with what Breeze's sizeFromContents() returns. Which is at this point still a 
size with the correct height of 32 px.

And then, as with 6.8.1, (rule.hasBox() || rule.hasBorder() || !
rule.hasNativeBorder()) matches and the size increased via sz = 
rule.boxSize(sz). But now not from the initial QSize(36, 20) (which doesn't 
come from Breeze), but from Breeze's QSize(52, 32), thus resulting in 
QSize(64, 44). And here we get a QSpinBox that is rendered too tall.


I don't have enough insight of all this to decide if Qt doesn't behave 
correctly here, or if Breeze should do something different. I'm not sure 
though if Breeze can know at all if QStyleSheetStyle::sizeFromContents() calls 
its sizeFromContents() (where we would need a modified QSpinBox height to get 
what we actually want) or if the default style class calls it, where the the 
size Breeze returns is rendered as-is.


However, I hope this helps either the Qt or the Breeze guys to figure out who 
is responsible here and who is free of guilt ...