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Using the latest source code, running on Linux Mint 19.3.

As of commit f5d78b2 by @jtanx, I find that I cannot use the Validate command. FontForge crashes with the message
fontforge: ../gdraw/ggdkcdraw.c:52: _GGDKDraw_CheckAutoPaint: Assertion `false && "No window should be drawing outside of expose calls"' failed.

To reproduce:

1. Create a new font.
2. Open a glyph and draw some random spline, e.g. with the pen tool. Note: I think it is relevant that the points are placed at non-integer coordinates.
3. From the main window, run Element > Validation > Validate...
4. A dialog box will appear, asking about how to handle the non-integer coordinates.
5. Whatever button you click, FontForge will crash.