I can't figure out how to make my transactions fill the full width of my monitor, the width appears to be fixed, and I've got a horizontal scroll bar.

The screenshot below is from transaction ME21N, in S/4HANA, running on SAP GUI client 770, to illustrate the issue.

There's over 50% of my screen unused, which would be super-useful for this t-code and many others.

Does anyone know what I need to do to "fix" this?

I have tried all the available themes, and can't seem to find any setting to modify this.

Thank-you in advance! :)