How do I set the header bar to collapse?

Add this to the left panel, at the top:

$meta
  header
    mode = minimal

DM me plz

https://perchance.org/tags-and-header-meta-data-example

If I'm reading this right, you probably want to override the CCS for the bar. I've seen this on a few pages. Best thing to do is find a page that does this see what their doing to make it happen. Copy the JS from the page and give it to any big name chatbot, it should be able to find where this happens, explain how it works and how to incorporate it into your gen.