View separator can be pulled beyond allowed bounds

**Describe the bug**
When moving a vertical separator line, it looks like it can be moved further than actually possible. In the process, the UI of the view being shrunk is distorted. When releasing the mouse button, the separator snaps back to the position.

**To Reproduce**
Steps to reproduce the behavior:
1. Open example template
2. Grab separator
3. Pull it to the left
4. Release

**Expected behavior**
The vertical separator can only be pulled until the position where will stop.
When releasing the mouse button, its position does not change anymore.
When the separator itself is not moving, no other UI widget is affected.

**Screenshots** (2x scale)

https://user-images.githubusercontent.com/708488/135751259-de1af402-59dc-4edd-9273-9a882de02816.mp4

**Desktop**
Windows 10 x64

Reproduce the behavior in the example template by dragging the vertical separator left, then trace the separator's drag handling and bounds calculation. Done means the separator stops at its allowed position during dragging, the neighboring view is not distorted, and releasing the mouse does not move it again.

Written by the indexing model from the issue text.