It's firefox 79 on ubuntu. The only unusual thing about my setup is that i have layout.css.devPixelsPerPx to 1.5, but the same thing happens when i set it to default.
I thought it was an add on, too, but it does the same in privaet mode without add ons and I don't have any FE modifications on lain.com... Really strange.
Found the issue. I have the default font for sans-serif set to Noto Sans CJK JP. If I change that back to the default (Deja Vu Sans), the issue goes away.
Figured it out. <textarea> by default has attribute cols="20" which defines it default/auto size, so it turns out it become bigger than container when used with certain fonts. No idea how exactly it calculates the size of it, but given that dividing textarea's effective width by cols gives somewhere around 15px while font-size is 14px, it seems that it might be using some japanese character as a base, so it's probably font-dependent.
ah... now i get it!1 it's the subject field that makes it grow, it also affects other inputs too, like search in the top and makes emoji reaction picker broken too.