opengl render size too small in X11 window

[Chris Marshall]

Dear Cygwin-

Using cygwin X11 under windows 10 with a high dpi display (3840x2160)
laptop resulted in far too small font and widgets using -multiwindow for
the XWin option.  If I set the windows setting display resolution to
1920x1080 and following the suggestion for the ~/.Xresources file from
https://stackoverflow.com/questions/42211564/cygwin-xwin-scaling-broken-when-using-windows-scaling-on-high-resolution-displa
I was able keep windows, widgets, and title/menu bars at a reasonable size
for reading.

However, when I display some OpenGL graphics content (using a FreeGLUT
window), then the visible OpenGL rendered area is about 2/3 the apparent
widget area of the X11 window.  If I try the same in a non-multiwindow Xwin
session then the rendered area is correct.  This problem is even worse if I
try with the full screen resolution.  At the moment I think I have narrowed
it down to the -multiwindow support for OpenGL contexts.

This is with a couple of versions of the freeglut libraries with no effect
and the current cygwin and cygwin X Server.

Thanks,
Chris

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