The split between the chain list and the params sat at a fixed 300 px,
wrong for a short chain and for a long one alike. It now rests under the
chain's rows, growing with the chain up to eight stages before the list
scrolls. Once the user drags it, it stays put; a double-click on the
handle fits it to the rows again.
GtkPaned sizes the fixed pane from its minimum height, which is only the
drag floor here, so the fit is done by hand from the natural height.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
