Can't resize channels in track visualization

### Bug description

In Audacity 3 I could resize channels in Track Visualization (so I could, for example, focus on one channel more and see things better on spectrogram and whatnot). I just tried it on 4 beta and all it does is select region and doesn't let me resize it. Either there's an option I'm missing or ~~could it be that it only works with classic layout~~? (edit: no, it doesn't work in classic workspace either)

### Steps to reproduce

1. Import stereo audio file
2. Try to resize one channel so it's smaller

### Expected behavior

I can resize stuff (Audacity 3.7.8)

### Actual behavior

I can't resize channels (Audacity 4.0.0-beta.2.261621301)

### Audacity Version

Audacity 4.0 beta 2

### Operating system

Linux

### Additional context

_No response_

Reproduce the issue in Audacity 4.0.0-beta.2.261621301 by importing a stereo audio file and trying to resize one channel in Track Visualization, then compare the behavior with Audacity 3.7.8. Trace the Track Visualization channel-resizing interaction and its region-selection handling; done means the channel divider can be dragged to change the relative channel sizes without selecting a region, in both classic and non-classic layouts.

Written by the indexing model from the issue text.