1. Change your main hand from right to left in options -> skin customization -> main hand
2. Hold a pocket computer in offhand (right).
3. Create a script that sleeps for a moment, then runs back.use(nil, "right")
4. Notice how it does nothing, try again, this time with back.use(nil, "left"). It will work amid the pocket computer being in the right hand.

When using "main"/"off" the correct hand is used. This issue occurs purely because it's incorrect to assume that the main hand is the "right" hand in ArgumentExt.kt:87

1. damn this is a big necro
2. I feel that at this point, correcting this issue may possibly be a breaking change considering that some people may be specifically using "right" for the main hand

1. not really, it's only been 3 and a bit months.
2. Just checked because I also was curious of this. On SC there are 0 cases of the .use method with "left"/"right" strings. Obviously this isn't explicit proof, but it leads me to believe that it won't be as major of a "breaking change" as you suggest.

realistically this should just be a boolean value anyways, false/nil = main hand, true = offhand

Not sure where you got "left" and "right" from, you're supposed to pass "main" or "off".

Suppose it wouldn't matter actually. Perhaps the skin customisation change isn't propagating to the fake player entity. Does the issue still happen after you relog? (For long enough such that the computer is terminated and really restarts)

My testing was done in a single player world. As for where I got the left/right from: https://plethora.madefor.cc/methods.html#org.squiddev.plethora.integration.vanilla.method.MethodsKineticActions-use(IModuleContainer)
I'll get back to you tomorrow on if the issue can be repeated on the SP world after a restart, as well as on the server