Problem drawing with Qt6 Fusion style - other styles working fine

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I have a subclass of QProgressBar that draws itself with its bar extending left or right from the center point (to show both positive and negative values). I developed it under Qt5, where it worked fine with all the available styles. With Qt6, however, the CE_ProgressBarContents portion of the control is not drawn consistently between styles. "Windows" and "windowsvista" are rendering it correctly, but "Fusion" is not. It almost looks as though Fusion is not interpreting the coordinates in QStyleOptionProgressBar correctly.

These gifs show the behavior of a test application in which the QProgressBar subclass is getting its value set by a slider. The only difference between the two gifs is the Qt style in use. The code is exactly the same.

windowsvista style:
vista-style.gif

Fusion style:
fusion-style.gif

The min/max values allowed by this widget are -256 and 255. Its paintEvent code is below.

Does anyone know what might be going on here? Was there some change in the Fusion style behavior between Qt5 and Qt6? Or a change in the intended use of QStyleOptionProgressBar? I have tested this on Windows and Linux, with Qt versions 6.4.2 and 6.5.2. The anomalous Fusion behavior appears everywhere.

const int currentVal = this->value();
  const int max = this->maximum();
  const int min = this->minimum();
  QStylePainter painter(this);
  QStyleOptionProgressBar opt;
 
  opt.initFrom(this);
  opt.minimum = min;
  opt.maximum = max;
  opt.progress = (currentVal >= 0) ? qAbs(max) : qAbs(min);
 
  // Draw the groove. This should simply be the same size as the outside dimensions of the widget.
  style()->drawControl(QStyle::CE_ProgressBarGroove, &opt, &painter, this);
 
  // Get the dimensions for the bar itself (within the groove). Depending on the active graphical
  // style, this may be several pixels smaller that the dimensions of the widget, so that there's
  // a narrow gutter inside the groove not occupied by the bar.
  opt.rect = style()->subElementRect(QStyle::SE_ProgressBarContents, &opt, this);
 
  const float percentOfWidth = (float)(qAbs(currentVal)) / (float)(max - min);
  const int left = opt.rect.topLeft().x();
  const int right = opt.rect.topRight().x();
  const int top = opt.rect.topLeft().y();
  const int height = opt.rect.bottomLeft().y() - top + 1;
  const int barWidth = (right - left) * percentOfWidth;
  const int midPoint = left + ((right - left) / 2);
  const int startPoint = (currentVal >= 0) ? midPoint : midPoint - barWidth;
 
  opt.rect = QRect(startPoint, top, barWidth + 2, height);
  style()->drawControl(QStyle::CE_ProgressBarContents, &opt, &painter, this);

const int currentVal = this->value();
  const int max = this->maximum();
  const int min = this->minimum();
  QStylePainter painter(this);
  QStyleOptionProgressBar opt;

  opt.initFrom(this);
  opt.minimum = min;
  opt.maximum = max;
  opt.progress = (currentVal >= 0) ? qAbs(max) : qAbs(min);

  // Draw the groove. This should simply be the same size as the outside dimensions of the widget.
  style()->drawControl(QStyle::CE_ProgressBarGroove, &opt, &painter, this);

  // Get the dimensions for the bar itself (within the groove). Depending on the active graphical
  // style, this may be several pixels smaller that the dimensions of the widget, so that there's
  // a narrow gutter inside the groove not occupied by the bar.
  opt.rect = style()->subElementRect(QStyle::SE_ProgressBarContents, &opt, this);

  const float percentOfWidth = (float)(qAbs(currentVal)) / (float)(max - min);
  const int left = opt.rect.topLeft().x();
  const int right = opt.rect.topRight().x();
  const int top = opt.rect.topLeft().y();
  const int height = opt.rect.bottomLeft().y() - top + 1;
  const int barWidth = (right - left) * percentOfWidth;
  const int midPoint = left + ((right - left) / 2);
  const int startPoint = (currentVal >= 0) ? midPoint : midPoint - barWidth;

  opt.rect = QRect(startPoint, top, barWidth + 2, height);
  style()->drawControl(QStyle::CE_ProgressBarContents, &opt, &painter, this);

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I've solved this. In earlier Qt versions, the style determined the orientation of the progress bar by checking the 'orientation' field in the QStyleOptionProgressBar instance. In Qt6, it seems this field has been eliminated and the bar's orientation is now captured in the 'state' member of the QStyleOptionProgressBar. Ensure that the State_Horizontal bit is set here; otherwise, the Fusion style will default to treating the progress bar as vertically oriented.

opt.state |= QStyle::State_Horizontal

opt.state |= QStyle::State_Horizontal

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