fastPathBottomToTop does not do much... It just reverse the line order.
https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/f5f0e7d98effd7217dded6ed9397b2cac1f787ad/platforms/Cross/plugins/BitBltPlugin/BitBltGeneric.c#L517

I foresee two cases where it could fail:

the fast operation by itself has a BUG (whatether BottomToTop or not)
some integer operations are carried on a larger integer type, transforming negative pitch into a large positive int

The later is possible because srcPitch and dstPitch are declared usqInt in operation_t
https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/f5f0e7d98effd7217dded6ed9397b2cac1f787ad/platforms/Cross/plugins/BitBltPlugin/BitBltDispatch.h#L98

usqInt is 64bits on ARM64, so far so good, unfortunately those variables are copied to 32 bits local temps in
https://github.com/OpenSmalltalk/opensmalltalk-vm/blob/f5f0e7d98effd7217dded6ed9397b2cac1f787ad/platforms/Cross/plugins/BitBltPlugin/BitBltInternal.h#L137

I think that it is the mistake, because pointer arithmetic in 64 bits will be performed with 64 bits size_t, and the uint32_t will just move the pointer a large amount forward (2^32-pitch) instead of backward (-pitch).

So I think that declaring the local srcPitch and dstPitch as usqInt instead of uint32_t might fix the bug.

I have no ARM64 at hand to play with, so up to you to verify the hypothesis.

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