Thanks for trying and for updating the other things. Just to be clearer, the two issues are:
The edit textbox (the post composer?) at the bottom of the sidebar panel is always below the bottom of the browser window and inaccessible, even with the css recalc.
The discourse behavior of hiding the custom header when the topics list is scrolled down causes the sidebar to stay too far down from the top (as in it is correctly position with the custom header shown, but doesn’t adjust when the header is hidden).
The header we added was via the Customize/Themes/Edit CSS/HTML - Desktop / Header section, i.e. here:
I did retry with the exact CSS you used but with no luck (with a slightly different height for our header) so we might be trying to recreate it differently.