1. Problem Screenshot:

See attachments "problem screenshot".

2. Steps to Reproduce:

a. Device setup condition:

• Use a Windows device
• In display settings, set system scaling to a non-integer value (e.g. 125%)
  If you cannot find the scaling setting, refer to the attached documents screenshot.

b. Code condition:

• Add the following code line

  QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
  

• Set Cache Mode in QGraphicsItem

  MyGraphicsItem::MyGraphicsItem(QGraphicsItem* parent)
      : QGraphicsItem(parent)
  {
      // or ItemCoordinateCache, the specific cache mode is not important
      setCacheMode(DeviceCoordinateCache);
  }
  

3. Source Code Analysis:

In "/qtbase/src/widgets/graphicsview/qgraphicsscene.cpp", I found this drawItemHelper:

void QGraphicsScenePrivate::drawItemHelper(QGraphicsItem *item, QPainter *painter,
                                           const QStyleOptionGraphicsItem *option, QWidget *widget,
                                           bool painterStateProtection)
{
// line 4423
const qreal devicePixelRatio = painter->device()->devicePixelRatio();

In QGraphicsScenePrivate::drawItemHelper, devicePixelRatio() is used instead of devicePixelRatioF().

devicePixelRatio() returns an integer by rounding down the ratio(1.25->1). devicePixelRatioF() returns a float preserving decimals.
This makes me wonder:

• For high-dpi displays with fractional scaling, is the integer ratio from devicePixelRatio() accurate enough?
• Should devicePixelRatioF() be used here to get the precise ratio with decimals?
• Or is there something else I need to set in my application code to properly handle high-dpi fractional scaling?

4. In conclusion

I would be grateful if Qt developers could validate the root cause and provide guidance.

To aid analysis, I have attached a demo project and screenshots demonstrating the problem. Please let me know if any other details are needed.