You may be right about the correct solution, but nobody actually uses any app long term with it straddling two displays, so the actual impact of this is not huge.

And DIPs have their own problems that I first encountered with WPF - rendering an application on a DPI that's not a neat multiple of what it was designed for means that lines and features don't necessarily line up with the pixel grid.

Depending how the app chose to handle this, it either causes blurriness or uneven and changing line widths as you move the window.