https://doc.qt.io/qt-6/coordsys.html says:

"When rendering with a one pixel wide pen the pixels will be rendered to the right and below the mathematically defined points."

So, QPainter::drawRect(0,0,... should result in visible lines. This is the case when using QPainter with a QWidget, but doesn't appear to be when using QOpenGLPaintDevice and QOpenGLWidget.

When x coordinate is 0, painting in to QOpenGLPaintDevice results in missing left edge as below

    painter.drawRect(0, 0, 99, 99);

See attached testcase.