QWidget::setStyleSheet() breaks cascade inheritance

JonB

I am stuck on inexplicable behaviour as a result of a QWidget::setStyleSheet().

I have a parent hierarchy which includes ancestor.setStyleSheet(something). Let's say it's {background-color: red;}.
I add child to parent.
At this point all is well: child and its descendants correctly inherit the ancestor's red color.
If I test child.setStyleSheet("") (i.e. empty string), all is well, nothing changes.
However, if I test child.setStyleSheet(" ") (i.e. anything, just a single space will do) it breaks the cascade inheritance: child and its descendants are no longer red!

I am aware this sounds bizarre, but it's deffo happening from just that change.

[The reason this is a problem, as I'm sure you'll ask, is: Qt Designer is used to produce the widgets. There every widget has a styleSheet property, which goes into the .ui file and produces a setStyleSheet() statement. If you don't visit the property it doesn't output any statemant and all is well. But if you happen to click into it you get a whitespace-only setStyleSheet() statement, and that's enough to break the run-time behaviour, even though you don't even know you have set a stylesheet.]

Of course I have tried to knock up a small example, but that doesn't exhibit this behaviour. The real hierarchy is large, and uses various widget types, and I don't know which bits of it cause this. [Note: I have searched, there is no code which manipulates any of the stylesheets after the generated construction code in setupUI().] If it is relevant it may be that I see this when QTabWidget, QScrollArea & QScrollBar are involved, though my tests with that still did not produce the problem.

Because I cannot produce a test, I'm desperately looking for any clue as to why child.setStyleSheet(" ") could possibly cause this where child.setStyleSheet("") does not??!! Presumably something, somewhere in Qt widget stuff is saying "if an element has no/only empty styleSheet allow cascade inheritance to proceed, but if it is not empty (even just a space) cancel all inheritance". According to me this behaviour should simply never happen? GULP :(

SGaist

Hi,

I don't have the exact answer but an empty string or a string with one space is pretty different when parsing.

I would guess that the space introduces an invalid style sheet.

JonB

@SGaist
Thanks, but it's not that. I mean if I have anything at all, yes including something perfectly correct for QSS, it stops inheritance from there downward, The space was only a minimal illustration!

kshegunov

I'm sad to be the hand that puts the stick in your wheel, but I believe this is a "feature" ... one that I also believe doesn't have a (reasonable) workaround. As far as my frail memory serves me, from the point where you set a stylesheet to the widget there's no more inheritance*. To have the correct style one has to set the whole stylesheet anew.
As for why space differs, I'd hazard a guess that QString(" ").isEmpty() is simply false, while if it were true the setStyleSheet() method just quickly returns doing nothing and/or simply clears the current stylesheet (thus allowing the parent one to be used).

* Because I believe there's no inheritance as such. If there's a stylesheet set it is used as such, if not then it searches if the parent has a stylesheet, no mixing, matching and inheriting style as you'd be used to from CSS.

Christian Ehrlicher

QWidget::setStyleSheet("") passes an empty string which means the style sheet should be removed. " " is not treated as empty (as it's not empty) but as style sheet.
btw: You can reset a property in the designer.

JonB

@kshegunov
As per @SGaist this sort of thing is not the issue. Please, it has nothing to do with spaces.

It only happens on one kind of node. There is a problem here.

JonB

All of you have picked up on the space. It has nothing to do with that. Please.

Just pretend I had written anything you like as legal QSS. Let's say child.setStyleSheet("QPushButton {} "). OK now?

I did say at the outset:

However, if I test child.setStyleSheet(" ") (i.e. anything, just a single space will do) it

Anything. Just not an empty string.

I know how inheritance works. I know it works in all situations than my problem one.

kshegunov

@JonB said in QWidget::setStyleSheet() breaks cascade inheritance:

It only happens on one kind of node. There is a problem here.

Then, if I'm wrong that is, I have no bloody clue.

Christian Ehrlicher

@JonB said in QWidget::setStyleSheet() breaks cascade inheritance:

It has nothing to do with that

Then take a look at QWidget::setStyleSheet(): https://code.woboq.org/qt5/qtbase/src/widgets/kernel/qwidget.cpp.html#_ZN7QWidget13setStyleSheetERK7QString (line 2728)

kshegunov

@Christian-Ehrlicher said in QWidget::setStyleSheet() breaks cascade inheritance:

Then take a look at QWidget::setStyleSheet()

Oh god, that's some mess ... but on the bright side it does seem to support my original hypothesis ...

JonB

@kshegunov said in QWidget::setStyleSheet() breaks cascade inheritance:
What hypothesis ?

kshegunov

@kshegunov said in QWidget::setStyleSheet() breaks cascade inheritance:

I'm sad to be the hand that puts the stick in your wheel, but I believe this is a "feature" ... one that I also believe doesn't have a (reasonable) workaround. As far as my frail memory serves me, from the point where you set a stylesheet to the widget there's no more inheritance*. To have the correct style one has to set the whole stylesheet anew.
[nonsense about the space]
* Because I believe there's no inheritance as such. If there's a stylesheet set it is used as such, if not then it searches if the parent has a stylesheet, no mixing, matching and inheriting style as you'd be used to from CSS.

Basically if you set a border style to the parent, but then set only background for the child, the parent's style is completely disregarded from the child's painting onward.

JonB

@kshegunov said in QWidget::setStyleSheet() breaks cascade inheritance:

where you set a stylesheet to the widget there's no more inheritance*

But that's wrong. There is inheritance. It works all over the place. QSS behaves similarly to CSS.

kshegunov

@JonB said in QWidget::setStyleSheet() breaks cascade inheritance:

But that's wrong. There is inheritance. It works all over the place. QSS behaves similarly to CSS.

Not according to my quick glance through the source.

JonB

@kshegunov
Hmmmmm.

Look, at this point, let me look again next week and make absolutely certain. I claim I am finding that inheritance works like it does in CSS, That's what "cascading" is about. I can set one style on a high node and a different style on a low node and they both appear. At this point let me check up and decide whether I am correct or I am going insane. :)

EDIT
https://doc.qt.io/qt-5/stylesheet-syntax.html#cascading

Cascading

Style sheets can be set on the QApplication, on parent widgets, and on child widgets. An arbitrary widget's effective style sheet is obtained by merging the style sheets set on the widget's ancestors (parent, grandparent, etc.), as well as any style sheet set on the QApplication.

Is that not in direct contradiction to your

Basically if you set a border style to the parent, but then set only background for the child, the parent's style is completely disregarded from the child's painting onward.

?

If you couldn't do this, whenever you set a stylesheet on some widget it would lose all your application- or window-wide styles you have set. It's fundamental to CSS/QSS. I don't know why people are saying stylesheets on different nodes don't merge.

kshegunov

@JonB said in QWidget::setStyleSheet() breaks cascade inheritance:

Is that not in direct contradiction to your

Yes, I checked, I'm wrong. Here's the test case:

#include <QApplication>
#include <QWidget>
#include <QLabel>
#include <QVBoxLayout>

int main(int argc, char ** argv)
{
    QApplication app(argc, argv);
    Q_UNUSED(app);

    QWidget main;
    main.setGeometry(0, 0, 100, 50);
    main.setStyleSheet("QWidget { color: blue }");
    main.setLayout(new QVBoxLayout());

    QLabel * child = new QLabel(&main);
    child->setText("Some text");
    child->setStyleSheet("QLabel { background-color: gray }");
    main.layout()->addWidget(child);

    main.show();

    return QApplication::exec();
}

JonB

@kshegunov
Phew! :)

The whole point of this question is that usually everything does merge, always has done. I am presently stuck in one situation in a complex design where if a node has *any kind of styleSheet set on it something then says that should terminate all previous styles. It should not. The question is why? :)

From your example, imagine that after

child->setStyleSheet("QLabel { background-color: gray }");

from then on all childs descendants do not now respect the main.setStyleSheet("QWidget { color: blue }"); from earlier on. (But do still respect child's QLabel { background-color: gray }.)

The behaviour, I think, will not lie in setStyleSheet(). Rather, it will be in the code which calculates the cascaded/inherited styles during rendering.

kshegunov

@JonB said in QWidget::setStyleSheet() breaks cascade inheritance:

The question is why?

Not the faintest idea. I know it's not too helpful, but the debugger is your friend here ...
(you may want to try GammaRay as well)

JonB

@kshegunov said in QWidget::setStyleSheet() breaks cascade inheritance:

but the debugger is your friend here

Not when one is writing from Python, so you can't step through Qt stuff....

JonB

I will spend another day next week trying to reproduce on a test example :) [I may have already found one odd case with QTabWidget.] Will report back.