mix setStyleSheet and setFont: wrong background - c++

Question

I merge several classes and functions into only two classes,
so it looks strange and ugly.
The problem that my class MyW at the end of constructor
set background to white, but it's child QLabel has background
from Page, not from MyW.
And question why?
I know that if I remove magic with fonts in MyW,
or call setStyleSheet at the begining like this:
setStyleSheet("border:none;background:#ffffff;color:#000000;");
I get the right result (white background),
but I can not understand why font make influence to background,
and why there is difference between set stylesheet in two steps,
instead of one?
#include <QApplication>
#include <QLabel>
class MyW : public QWidget {
public:
MyW(QWidget *parent) : QWidget(parent) {
setStyleSheet("border:none;");
setFont(QFont{"Arial", 42});
setStyleSheet(styleSheet() + "background:#ffffff;color:#000000;");
auto lbl = new QLabel{"AAAA", this};
lbl->ensurePolished();
}
};
class Page : public QWidget {
public:
Page(QWidget *parent) : QWidget{parent} {
setStyleSheet("background:#f0f4f7;");
auto item = new MyW{this};
}
};
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
Page p{nullptr};
p.resize(400, 800);
p.show();
return a.exec();
}
Update: I removed all not important parts, like layouts, QApplication::setFont and so on.

Answer 1

I am sure that reason is not in auto font = QApplication::font();, but
in QWidget::setFont call. You can check it, for example, by replacing font stuff with:
QFont font;
font.setPixelSize(42);
setFont(font);
The "magic" behind this is cache that is used inside Qt to deal with style sheets.
You can find hint in qtbase/src/widgets/styles/qstylesheetstyle.cpp, look at usage of
static QStyleSheetStyleCaches *styleSheetCaches = 0;
Qt does not use QWidget::styleSheet property directly,
it parses it and caches result.
There are several triggers for parsing(re-parsing) of QWidget::styleSheet:
Call of QWidget::ensurePolished (it is done automatically when your widget
becomes visible for the first time);
Call of setStyleSheet but only in the case if your widget called QWidget::ensurePolished at least once;
Call of QWidget::setFont or QWidget::setPalette (only if your widget does not have an empty styleSheet).
In your case your problem is a combination of (1-2) and (3):
after force caching of the parsed stylesheet via QWidget::setFont your widget is still not "polished",
so the next call of setStyleSheet does not update the cached style sheet that was created on setFont step,
so on the step with lbl->ensurePolished(); you actually have a style sheet with "border:none;" plus font, plus background of parent.
You can call this->ensurePoslished() before lbl->ensurePoslished() to fix this issue or as suggested by #William Miller use stylesheet to set font,
or place setFont after all calls of setStyleSheet

Answer 2

Per the docs on QApplication::setFont(),
Warning: Do not use this function in conjunction with Qt Style Sheets. The font of an application can be customized using the "font" style sheet property. To set a bold font for all QPushButtons, set the application styleSheet() as "QPushButton { font: bold }"
Since they explicitly warn against this I would expect there is an inheritance conflict when using style sheets in conjunction with the application level default font, so the line
QApplication::setFont(font);
and subsequently,
auto font = QApplication::font();
May not produce the expected behavior. Their alternative is set the application-level styleSheet() for your class, i.e.
setStyleSheet(" MyW { font-family: 'Garamond' }");
So it seems the short answer is that they don't support it.
The reason it makes a difference "between set stylesheet in two steps, instead of one" is not because you are setting the style sheet in two different steps but because you call QApplication::font() between the steps and as this is unsupported behavior, it is producing the unexpected result.