Compose soft word wrap (HTML format) does not use window size
In the Compose window, composing in HTML format with the Formatting Bar visible: The soft word wrap only wraps to the maximum of the window width or the Formatting Bar width. Since I typically compose with a window narrower than the Formatting Bar, this causes the right side of my message to be hidden.
Turning off the Formatting Bar returns word wrap to its normal function. This is not a solution, of course; only a work-around.
As far as I know, this started with version 115.0.1; at least, I did not notice this before, and I use Thunderbird on a daily basis.