[Interest] QComboBox is positioned incorrectly in a layout on macOS

[Indi Kernick]

I posted about this on the forums
<https://forum.qt.io/topic/105191/why-isn-t-a-qcombobox-positioned-correctly-in-a-layout/10>
and
it was recommended that I post a message here.

A QComboBox is shifted by a few pixels when it's put in a layout. This
causes the QComboBox to overlap neighbouring widgets. The issue can be
reproduced using this example:

#include <QtGui/qpainter.h>#include <QtWidgets/qcombobox.h>#include
<QtWidgets/qboxlayout.h>#include <QtWidgets/qmainwindow.h>#include
<QtWidgets/qapplication.h>

template <typename Base>
class Rectangle : public Base {
public:
  explicit Rectangle(QColor color)
    : color{color} {
    Base::setFixedSize(100, 20);
  }

private:
  QColor color;

  void paintEvent(QPaintEvent *) override {
    QPainter{this}.fillRect(Base::rect(), color);
  }
};
int main(int argc, char **argv) {
  QApplication app{argc, argv};
  QMainWindow window;
  QWidget central;
  window.setCentralWidget(&central);
  QHBoxLayout layout{&central};
  layout.setSpacing(0);
  layout.setContentsMargins(0, 0, 0, 0);
  layout.addWidget(new Rectangle<QWidget>{Qt::red});
  layout.addWidget(new Rectangle<QComboBox>{Qt::blue});
  window.show();
  return app.exec();
}

You can easily switch the between QWidget and QComboBox using the
template. Here's a screenshot of what I'm seeing with the combo box:

[image: combobox.png]
The blue rectangle (QComboBox) is overlapping the red one (QWidget). I
expect to see this:

[image: widget.png]
The two widgets are sitting next to each other in the layout.

I'm hoping to understand why this occurs. This seems like a bug to me. A
workaround that I'm yet to try is to offset the painting of the combo box
to compensate. That's not ideal though. I tried adding a margin using
stylesheets but that didn't do anything. Perhaps there's a better
workaround or even a fix?

I'm able to reproduce the issue on *Qt 5.13* and *macOS 10.14.6*.
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