I’ve finally figured it out.

Whipped up a user script to correct this horrible design flaw that came out with yesterday’s update.

Script and instructions are up on my site: https://wuffleton.com/code/tumblrsidebar/

I’m not responsible if this script burns down your house or starts a nuclear war, but it should at least work with Chrome and Firefox (possibly others, but untested)! Hope this is of use to some of you :3