A high-end code editor for GameMaker: Studio / GameMaker Studio 2 · By YellowAfterlife
(I think the video explained it well)
This is fixed by resizing the window horizontally to any extent - unfortunately, the way DOM resize event works, the code editor does not take notice of size changing when side panels pop in/out.
What about the little vertical line that still exists in the middle despite of having resized the window?
(I gestured it on minute 0:13)
Is it intentional perhaps?
That's print margin, you can tweak/disable it in code editor settings