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QWidget::setStyleSheet() breaks cascade inheritance



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

    1. I have a parent hierarchy which includes ancestor.setStyleSheet(something). Let's say it's {background-color: red;}.
    2. I add child to parent.
    3. At this point all is well: child and its descendants correctly inherit the ancestor's red color.
    4. If I test child.setStyleSheet("") (i.e. empty string), all is well, nothing changes.
    5. 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 :(


  • Lifetime Qt Champion

    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.



  • @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!


  • Qt Champions 2017

    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.


  • Qt Champions 2019

    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.



  • @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.



  • 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.


  • Qt Champions 2017

    @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.


  • Qt Champions 2019


  • Qt Champions 2017

    @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 ...




  • Qt Champions 2017

    @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.



  • @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.


  • Qt Champions 2017

    @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.



  • @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.


  • Qt Champions 2017

    @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();
    }
    


  • @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.


  • Qt Champions 2017

    @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)



  • @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....



  • 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.



  • I just wanted to report back in case anybody thinks I am slacking....

    Spent another day of frustration :( In my real-life, I have something like hierarchy ... > QTabWidget > QScrollArea > .... Higher up sets stylesheet like QScrollBar { ... }. I am finding that setting any kind of non-empty [including nothing related to scrolls/tabs] setStyleSheet { ... } on either the QScrollArea or on its direct parent (like QTabWidget here) stops inheritance of the scrollbars. Putting in setStyleSheet { ... } anywhere else (e.g. higher up than direct parent) does not stop inheritance....

    ...But of course, try as I might, I cannot reproduce this behaviour on any kind of standalone... :( It's proving impossible to recognise what the difference is.

    One thing: I just happenstanced across @sierdzio's comment from Feb 2019 at https://forum.qt.io/topic/100083/qt-how-to-prevent-child-qdialogs-or-qwidgets-from-inheriting-parent-stylesheet/2:

    If you want to stop subwidgets from inheriting the style, call setStyleSheet("") on widgets which should have default style sheet.

    [Leave aside for the moment that I find setStyleSheet("") has no effect/does not cancel inheritance, it must be setStyleSheet(<something-here>), even if it's just " ".]

    Is @sierdzio around? I need him or somebody to give me an actual reference to where they have seen this in the docs, please? It does not seem to me things work like that, at least not in the cases I have tried (and see @kshegunov's sample code above), but there does seem to be a possible relationship between this statement and (some of) my issue, so I really need to pursue....


  • Moderators

    Heh, yes I am around. I didn't respond earlier because I thought I had an answer... then rechecked the docs and it turned out I remembered it incorrectly.

    So, I don't have an explanation for you. Perhaps it is a Qt bug (quite probably it is, in fact!), or you are doing something wrong (seems unlikely, from what you describe). I can only offer some hints that might help:

    • do you break parent/child relationship on your widgets at any point (using QObject::setParent() or something similar)? It could break QSS inheritance chain, too
    • I have recently stumbled upon some widgets which do not work with QSS at all if they have custom QPalette set. I don't have a full list, from memory I'd say it was at least QLabel and QLineEdit, probably more. So if you do call setPalette() anywhere - this could be your problem
    • there is a way to avoid setting custom stylesheets for particular widgets. If you want some widget (let's call it WidgetA) to have different look, but you also want to have only one application-wide QSS file, then: pick some property name and set it WidgetA->setProperty("myCustomName", true);, then in your QSS file, you can create special rules only for widget(s) which have this property: QWidget[myCustomName] { background-color: #FFFFFF; border: 1px solid #E9EDEF;}. It works, and as a bonus you get all your style code in a single, global place :-)


  • @sierdzio
    Hi there, thanks for replying :)

    • No reparenting, nothing going on in code like that (bit I will recheck tomorrow). The layout is designed from Creator, not manipulated in our code.
    • Nothing like palettes or special widgets.
    • Yep, I did dynamic properties in my previous project. Not at issue here. I have done a lot of CSS for my sinds, so I do know what it's all about.

    So can we be clear, please? Are you withdrawing your:

    If you want to stop subwidgets from inheriting the style, call setStyleSheet("") on widgets which should have default style sheet.

    I have not seen Qt says it would do that. Unless it says it does I would not expect it to --- it would break the simplest case of defining a global set of styles and then occasionally doing one thing on an element if that cancelled all inheritance. My "standalone" tests do not reveal such behaviour.

    Yet my problem cases are behaving as though that does happen, in unrepeatable circumstances. That is why I ask. Something seems to be going "if such and such an element has any stylesheet of is own, cancel inheritance from its ancestors at this point and down to descendants". When it should not.

    As I have said elsewhere, it may have something to do with QTabWidgets & QScrollBars. I have already discovered that "containers" like these do not behave as one would expect for inheritance, I may make a separate post tomorrow showing that. EDIT See https://forum.qt.io/topic/111137/stylesheet-inheritance-and-container-widgets.




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