Adder position doesn't update when scrollable container is scrolled

The Annotate/Highlight toolbar normally moves with the selection when the document is scrolled, but this doesn't work if a scrollable container within the document is scrolled, rather than the whole document.

**Steps to reproduce:**

1. Select some text in Via's video player
2. Scroll the transcript

**Expected result:** The adder toolbar should appear in step (1) and move with the selection in step (2)
**Actual result:**

The adder toolbar position stays fixed in step (2), becoming detached from the selection: