Pin logical position of the ListViewController while live-resizing.

[th-in-gs]

This allows the user to resize the window without losing their position in the timeline.

I'm a habitual window mover and resized, and it was driving me crazy how timelines would jump around when I moved or resized them.

See comments in the source changes for more details on how it's done.

I think this is a pretty idiomatic way to do this - though it's been probably 15 years since I've properly messed with an AppKit scroll view (as opposed to a UIKit one, which is pretty different)... The change is at least reasonably well self-contained.