It seems that the latest update or so of Discourse decided to restrict the width of the button toolbar to half the screen* width, and add a scroll bar.
The previous solution was better, where all the buttons actually fit, allowing the text box to be a bit bigger than the preview box.
I propose that the max-width be set to something like 75% for the main text box. That way not only do the current buttons fit, but you future proof it for a bit longer if extra buttons get added.
I don’t know if this is a theme change or a underlying code change. I will look into seeing if I can fix it with custom CSS.
*Technically it would be “content width,” but, in the situations where it matters, the content width and screen width are basically the same.