Enable usage of left and right margin.
This enables usage of set left and right margin.
Enabling this mode also disables support for double height or width font rendering (e.g .set double width line, set double height line top half , etc) and restores each line to the state as if set single width line was called for it.
Disabling this mode restores the scroll region to be the full terminal width.
🛈 xterm before version 338 had scrolling artifacts when double width characters crossed the boundary of the scroll region.