How to interactively stage changes (akin to git add -p) ?

[anentropic]

I saw this issue #531 so apparently the feature exists

I opened the GitLens control panel and selected a file with changes

I get a nice side-by-side view of the diff

But I could not work out how to use it

There's a ... menu in the top right corner that has an option "Stage selected ranges" which doesn't seem to do anything. But maybe I don't have any ranges selected? Can't tell how to do that.

There are -> buttons in the gutter between the two columns beside each change. Clicking that seems to undo the change into the right-hand column. Not sure what that represents

Whatever I do so far, nothing shows as staged in the left-hand GitLens control panel

How do I use this? thanks!