While I had been able to build at r259630, I cannot build at r260564, to which I just updated. The build error is: ../Source/WebCore/testing/js/WebCoreTestSupport.cpp:34:10: fatal error: JSInternals.h: No such file or directory 34 | #include "JSInternals.h"
I did not try to bisect the commit which caused this regression, I chose semi-randomly r260336 and I can build that one without any issue, thus it's something in between, which broke the build.