QPen's default join style is unfortunate for High-DPI mode.

This was noticed on the Mac with HighDPI in which a rectangle drawn with a pen of 1 width (the default) renders with a pen of 2. The rectangle corners are inconsistent. The right corners render with a rounded edge but the right corners are square. When the pen width is set higher all 4 corners are rounded. The corners with only 2 pixel pen width should be consistent.