The problem with environment variables is that it doesn't support dynamically adding and removing monitors and changing the scale factor of an existing monitor.

The problem with environment variables is that it doesn't support dynamically adding and removing monitors and changing the scale factor of an existing monitor. Also personally I think integer (if any) coordinate scaling, and fractional font-only scaling (or fractional blurry server-side scaling) is the only way to support fractional DPI scaling without breaking the fundamental assumption that integer-aligned pixel rectangles don't overlap or partially cover pixels. Qt's fractional coordinate scaling breaks much of current theming and app painting algorithms.