I did some testing to see why sometimes I see horizontal scrollbar in the chat on my games. Not even a character width is being scrolled. The scroll bar just appears and I can scroll a few pixels or something like half a character.
at game http://online-go.com/game/933756 I just spammed text that approached the width of my chat. text wrapping worked correctly.
at game http://online-go.com/game/936009 I spammed shared variations with different lengths that also approached the width of my chat. Text wrapping breaks when there is just a single character too much in the shared variation name. Putting 2 characters “too much” works correctly and the whole variation name is moved to another line.
This should be different amount of characters for each different window/chat widths.