Opening a 2.4.2 project with stereo tracks into 3.x gives the stereo tracks two Clip-handle drag-bars

[petersampsonaudacity]

Describe the bug
If you have a 2.x AUP project with stereo tracks and you then open/import that project in 3.x then you will see that each stereo track has not one but two Clip-handle drag-bars (one per channel). The second one is basically totally redundant.

This, in particular, becomes an issue if the user then collapses the tracks due to Issue #2708
as then no audio is visible, just the drag-bars.

To Reproduce
Steps to reproduce the behavior:

1. launch 2.4.2
2. get some stereo tracks
3. Save the project as AUP
4. close Audacity
5. launch 3.1.3 (or 3.2.0)
6. Open or import the AUP project from step 3
7. Observe: each stereo track has two Clip-handle drag-bars

Expected behavior
The right channel drag-bar is totally redundant (though it can be used to move the clip) and so should not be there in this use case.

Screenshots

User image from Forum post

Additional information (please complete the following information):

• OS: Windows 10 - but assume all OS
• Version: Audacity 3.1.3 and latest alpha master for 3.2.0

Additional context
The symptoms of this were originally reported by a confused Forum user:
https://forum.audacityteam.org/viewtopic.php?f=46&t=124445&p=448137

I assume that the 2.4.2 project that they had consisted of collapsed stereo tracks which worked fine in 2.4.2 with visible audio - but on opening the project in 3.1.3 all the user confusingly saw was drag-bars - see "Screenshots" above.

Flagging @Tantacrul as a UX / design issue

[petersampsonaudacity]

The workaround is simple if a little tedious as you can mix and render each track - BUT

a) this has to be done one track at a time,
Note that fixing: Proposal: add a new Tracks menu command for rendering tracks #2418
would make this a simpler single-pass task

b) if the user has any settings for Pan/Gain in the TCP or Envelopes on any of the tracks then those will we rendered, incorporated and lost.

[LWinterberg]

Is this different to #2618?

[petersampsonaudacity]

Is this different to #2618?

Ahhh - you're right @LWinterberg - sorry about that about that