Incorrect scaling behaviour if Windows settings changed

[kmcnaught]

If you change:

• what multi-monitor setup you have
• which monitor is primary (in a 2 monitor setup), or
• what UI scaling you have in the Windows settings

then Optikey has various incorrect behaviours until you have restarted your computer.

This is documented here: https://github.com/OptiKey/OptiKey/wiki/Multiple-monitors

but I raise it as an issue in case anyone has any suggestions on how to fix, and also as a future reminder to myself not to test anything without a full restart, and to document some of my findings.

The issue is that Graphics.DpiX and Graphics.DpiY are fundamentally incorrect until a restart (i.e. they keep the previous value). These are currently computed with:

    var desktop = g.GetHdc();
    DpiX = PInvoke.GetDeviceCaps(desktop, (int)DeviceCap.LOGPIXELSX);
    DpiY = PInvoke.GetDeviceCaps(desktop, (int)DeviceCap.LOGPIXELSY);

I have also tried

    var dpiXProperty = typeof(SystemParameters).GetProperty("DpiX", BindingFlags.NonPublic | BindingFlags.Static);
    var dpiYProperty = typeof(SystemParameters).GetProperty("Dpi", BindingFlags.NonPublic | BindingFlags.Static);

    var dpiX = (int)dpiXProperty.GetValue(null, null);
    var dpiY = (int)dpiYProperty.GetValue(null, null);
    

and, when I have access to the mainWindow:

    PresentationSource source = PresentationSource.FromVisual(mainWindow);
    double dpiX, dpiY;
    if (source != null)
    {
        dpiX = 96.0 * source.CompositionTarget.TransformToDevice.M11;
        dpiY = 96.0 * source.CompositionTarget.TransformToDevice.M22;

        Log.Info($"MainWindow FromVisual: dpiX={dpiX}, dpiY={dpiY}");
    }

All three methods exhibit the same incorrect behaviour. I have not been able to find any fix with my googlings.