Describe the bug
Label regions not visible in the timeline when annotating VideoRectangles on videos.

To Reproduce
Steps to reproduce the behavior:

1. Go to labeling interface for a task
2. Scroll down in timeline
3. See the jump between region numbers (some are skipped)

Expected behavior
I should be able to see all numbered label regions in the timeline when scrolling, e.g. if the bottom visible region number is 3, when I scroll the top visible region number should be 4

Screenshots

Then scroll once:

Environment (please complete the following information):

• OS: Windows 11
• Browser: Microsoft Edge (persists for Chrome also)
• Label Studio Version: 1.15.0 (happened with previous version as well)

Additional context
Zooming out with CTRL+Scroll reveals the blank section beneath the timeline (as shown in the screenshots), and I can't resize the timeline "window" vertically as with the other "windows"/tabs in the interface.