If anyone runs into this, the way I fixed it was
1. remove all responsive structure from parent css. There are ways you can bypass editing the parent css – either not importing parent css and instead copying into the beginning of your own child css, or via advanced design plugin.
2. simply add min-width: 1000px to #page in the global css.
well, looks like by cleaning out the responsive structure and combining css from parent, I have managed to keep the sidebar stationary. BUT, now the background is also fixed. How to let that go as wide as required?