This is a Windows issue, not WDL, so unfortunately the bug cannot be reported to Cockos.
When I first started with the JS_Composite function, I also tried blitting from overlapped windows using GDI, and encountered the same problem. I then read somewhere that this behavior changed sometime around Windows XP: In the olden days, you could blit hidden parts of windows, but not in recent versions of Windows. I am actually surprised that it works (again) in Windows 10.
Perhaps it has something to do with windows styles? (Perhaps check whether the "overlapped" style has any effect?)
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