How to make the TOC panel horizontally expandable and shrinkable

[BostonKevin]

Is there any way to make the Slate TOC panel expandable such that the other panel shrinks in proportion to the TOC panel's expansion?

After making a change to screen.css.scss I was able to expand the width of the TOC by dragging the bottom left corner but the right panel did not shrink - it was overwritten by the TOC.

Any help appreciated.