if you fill up the Workbench like this:
64 64 64
64 64 64
64 64 A
where A is any number of item except 64 and then trying to fill in a stack by shift right clicking it causes a crash instead of filling up the last slot

Well, please reopen if you can reproduce this on the most recent version like 1.18.

Uhh, I don't think Mouse Tweaks does anything on shift-right click? Are you sure it's not some other mod?

Yeah, shift left click, my bad

Do you mean shift-left click drag or just shift-left click?

Either way Mouse Tweaks shouldn't be doing anything weird here. Please post the crash log.

Could this be the issue, just happens to be dragging when doing these options? Here is the crash.
https://gist.github.com/tgstyle/8475bf9e860a54068db7926cc3f33e9c

Can you reproduce this every time? Which container? Could you record a video? Could you try with just Mouse Tweaks and no other mods?

Yes it happens in any container when left click dragging. I think it may be related to FastWorkbench, I am testing it now.

It's not FastWorkbench, I tried a fresh install with just Mouse Tweaks on Forge. Holding left click while doing it once seems fine to empty slots, but then picking up a stack and left click dragging over existing items while staying in the same GUI crashes. I tried with just a MC chest.

Also shift-left click dragging from inventory to a chest slot that is not yet full crashes.

So clean 1.14.4 with just Forge and Mouse Tweaks, and you can reproduce it? Could you record a short video showcasing the issue?

Correct, 1.14.4 with just Forge and Mouse Tweaks.
https://www.dropbox.com/s/t9dg7qn5jrdfb8a/Minecraft%201.14.4%202020-03-31%2022-02-28.mp4?dl=1

Thanks, I will look into this.

I can't reproduce this crash. Here's a video of me doing the same movements as you: https://streamable.com/6hfnsu

Were you holding the LMB the whole time?

Right at the very end of the video dragging right and then left while holding the LMB is when it crashed. I still cannot get it to reproduce yet.

Bizarre thing is, I can't get it to reproduce today either. It could be done very easily. I will keep testing and let you know the results.

Were you holding the LMB the whole time?

I tried to decipher from the video when you were releasing the LMB and release it at the same moments.

In the end of the video I let go of LMB to put down the chest stack, then hold it again on the empty slot and drag onto the rightmost chest slot.

That should have triggered it. I don't know what would have changed in the last few days, unless there was a library update.