setStyleSheet on QWidget does not have effect

[skebanga]

I have an OuterWidget which has a QGridLayout and a single widget, InnerWidget, within:

class OuterWidget : public QWidget
{
    Q_OBJECT
public:
    OuterWidget()
    {
	    InnerWidget* innerWidget = new InnerWidget();
	    QGridLayout* layout = new QGridLayout;
	    layout->addWidget(innerWidget, 0, 0, 0, 4, Qt::AlignTop);
	    setLayout(layout);
    }
};

InnerWidget does nothing other than set its background-color to black:

class InnerWidget : public QWidget
{
    Q_OBJECT
public:
    InnerWidget(QWidget* parent) : QWidget(parent)
    {
	    setStyleSheet("background-color: black");
    }
};

My expectation is for the widget to be displayed with a black background, but it doesn't:

If, however, I set the style sheet on the parentWidget, then it works as I expected:

class InnerWidget : public QWidget
{
    Q_OBJECT
public:
    InnerWidget(QWidget* parent) : QWidget(parent)
    {
	    parentWidget()->setStyleSheet("background-color: black");
    }
};

According to the documentation:

One consequence of this is that setting a style rule on a widget automatically gives it precedence over other rules specified in the ancestor widgets' style sheets or the QApplication style sheet

This suggests that setting a style sheet on a widget itself should work, but this doesn't seem to be the case.

Why does it work when I set the parent widget's style sheet, but not for the widget itself?

[JonB]

@skebanga
Is a possible answer to both you & @Smeeth in https://forum.qt.io/topic/91917/setting-the-background-color-of-widget-inside-another-widget:
A plain QWidget with nothing on it does not show its background (if it's a parent it has something on it)?

[SGaist]

Hi,

Add: setAutoFillBackground(true); in your InnerWidget constructor and you should be good to go.

[skebanga]

@JonB that is not correct

[skebanga]

@SGaist said in setStyleSheet on QWidget does not have effect:

setAutoFillBackground

From the documentation:

Warning: Use this property with caution in conjunction with Qt Style Sheets. When a widget has a style sheet with a valid background or a border-image, this property is automatically disabled.

[SGaist]

I don't know whether a background color qualifies for that warning.

[skebanga]

@sgaist Fair enough.

Do you have any idea why setting the stylesheet on the parent works?

Would you say that setting a stylesheet on the parent and having it cascade correctly to the child, whilst setting it on the widget itself does not work is a bug?

[mrjj]

Hi
Cusom Widgets and stylesheets can be a bit funky together.
I suspected it would turn off background drawing for other reason than using images properties but did not verify it/test it.

However, always painting with the style seems to fix it.
Its mentioned somewhere in Docs, but couldn't find it again :)

class InnerWidget : public QWidget {
  Q_OBJECT
 public:
  InnerWidget(QWidget* parent) : QWidget(parent) {
    setStyleSheet("background-color: black");        
  }
 protected:
  virtual void paintEvent(QPaintEvent* ) override {
    QStyleOption opt;
    opt.init(this);
    QPainter p(this);
    style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);
  }
};

It should then be black.

[Smeeth]

Hello everyone, thank you for the answers.

@mrjj You are correct, I also just found this page that describes exactly what you are saying about overridding the paintEvent class.

Thank you everyone!