Can't see some regions in timeline when scrolling down

**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.