fix bug where opening the side pane causes messages to become too tall

[cousincrusher]

repro:

• wraparound OFF
• open chat with long messages (multi bot might help? idk)
• open sidepane, then close it
• messages are way taller than needed because the current logic cannot detect when the message box's size goes down, only when it goes up!!!

fix used:

• instead of observing changes in the entire message box's size, including the element we add a min-height to itself, we observe changes to the text field only
• we have to add + 1em or it's not accurate, check that it works well on other PCs than mine

[sceuick]

I included this commit in another PR.