Clickable borders that collapse comment trees
For an example, I guess, it's best to look at the new Reddit layout, which finally added this feature; and by now it actually works well.
For the old layout here's a good implementation in Firefox I'm still using.
A comfortable width for the clickable area should be around 14-16px. The new Reddit design uses two layers, though, prioritizing style--a visible thin line and a transparent clickable area.