An “expand” option to view all the comments will be troublesome. Very large comment may not fit on the screen. A comment near the bottom won’t be visible while looking at nodes above the comment reference. A comment in the middle of nodes will cover referenced nodes while expanded.
Always having a scrollbar would indicate there is more text and provide a way to access it.
Allowing scroll wheel all the time is going to cause issues (conflicts with pan, do you scroll all comment nodes?). Allowing scroll wheel when node is selected will seem kind of arbitrary, but allows the functionality without causing conflicts. Only allowing scroll wheel when there is a single item selected makes it even more arbitrary, but alternative is to scroll all selected comment windows at the same time and you can’t pan if you happen to have a comment selected with a group of nodes.
While jumping back to the top of comment on de-select makes sense, there will probably be times someone wants to see the middle/bottom of comment while making changes to code, so leaving at current location is probably better. People can manually scroll back to the top using scrollbar if that is what they want.
Another question: does using the scrollbar select the node? Probably? I can see times I might have some nodes selected and need to scroll down to read more comments before doing anything with the selection, but it probably makes more sense to select the comment when scrollbar is used so scroll-wheel is activated for the comment.