[thelegendaryof]

Well then its indeed wrongly implemented (not technically bugged) in that case - keep in mind this isnt WoW but a completely other Software - they're just both using Lua and Eso might have been inspired (to whatever dimension) but it is not a 1:1 copy of the WoW-API. Thats especially true for any UI related functionality.

I cant check the behavior myself right now, as I dont have access to a PC atm (I'm writing from my Phone). But there are a few different solutions to that kind of missbehavior. Hack the ChatWindow and correctly calculate the visible Lines for yourself with your own function - that is the most "solid" solution which probably wont break anything else. Another would be hooking / overwriting the global GetNumHistoryLines - which is bound to break many stuff - as you dont know how its functionality is intended by the Developers and last but not least contact the Support/Developers and ask them how its intended to work, and that you found that its implementation in the ChatWindow isnt correctly taking the visible line dimension into consideration and as such leads to a to early/late display of the scrollbar.

All in itself its however pretty useless to complain about it here. Either find a solution for yourself or contact the developers / support like I've written in detail above.

Cheers