Got a window filled with black when attempting to start freeCAD's portable app (v. 0.21.1).
Symptoms are described here:
https://forum.freecad.org/viewtopic.php?t=36087&start=40
As discussed in the FreeCAD forum, renaming FreeCAD's bin/opengl32sw.dll to opengl32.dll fixed the issue. The Path to the file is something like ...\PortableApps\FreeCADPortable\App\FreeCAD\bin
Hope that helps, thanks!
This can happen with multiple apps if your system doesn't have a proper graphics chipset/driver, typically on laptops. You can add that same opengl32.dll file into apps like Blender (if I recall correctly) to get them to work as well. It's downloadable from Mesa3D.
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