@AltCode now this is simply stunning! OMZ, tis so clever of you, egads! ta bigly
fwiw, initially i did a double-take as i made my first attempt to narrow my lhs, coz whoa, it was the rhs pane that reacted. huh? then i realised; prolly my Instance has for whatever stylistic reason flipped the lhs & rhs panes around. i've never used v's Masto Instance before, but to answer my question, i logged into it, & yep, they're reversed. so now my lhs is --rhs-panel-width: 150px;, & my rhs is --lhs-panel-width: 300px;, & all is fine & dandy in the world [apart from all the very many things that are lousy & rotten, ofc ].
many thanks again.
unpredictable, unprecedented, unrequited
so many very clever peeps here. one would think that whipping up a quick infinitely-nestable tst would be a real doddle. shirley nobody could doubt this?