Success!

Sort of....

I've been looking for something like this for years, finally had inspiration and found a solution this morning that might be acceptable:

Options > Advanced > Style area pane width...

Set it to, say, 10cm. Close options and then manually
position as you like by dragging

Double bonus is it can help a little in organising styles (although i feel this could have been implemented a little better)

Gonna use this all the time now.