Is there a tweak that can be made to table-hover that will cause it to highlight all rows associated with a merged first column. Alternatively, can it highlight the first column when hovering on the second row
example as currently configured: only the first row in column 2 and 3 are highlighted
example when hovering second row:
Sounds like this calls specifically for a shout-out to @Mohammad (though perhaps you can get some ideas from someone else with stronger css skills than the average bear 
)
It will be interesting to see if this could be done. I’m not very optimistic, at least not with a pure CSS solution. I don’t think the sort of CSS selectors that would be necessary actually exist.
While I’m sure we could figure out a JS mechanism for this, it also sounds fairly complex.
@Scott_Sauyet one approach may be outside tiddlywiki altogether, I tend to use browser extensions that let me copy all or parts of tables, they can do this highlighting for me. It all depends on to what purpose you want it. If it is just highlighting, there are other options to on hover.
I think the question is specifically about hovering, though. Simple row hovering is trivial with CSS:
tr:hover {background: #cfc;}
As seen here: Beatles_ hover.json (401 Bytes)
But extending this to capture row-spans or, worse, nested row-spans, seems like a difficult problem.