like #29
just made worse by other mod's GUIs

This happens with Backpacks! v3.1.8 for 1.11.2 aswell

Some mods do weird things with their GUIs which causes issues for TrashSlot, as it expects everyone to follow vanilla patterns. In this case it seems like the size is dynamically changing based on your window size. Just drag the slot to the position you want and it'll stay there.

Looks like I misunderstood this issue, but it should be fixed in the latest version now (although some mod GUIs are likely still broken, they will no longer share the same slot settings as other GUIs).