There's somthing wrong with mute, waveform, and multi track

[TRA]

Steps to reproduce:

1. Launch Shotcut
2. Open a video with some audio or audio file.
3. Move the entire thing or a portion of it to the Timeline.
4. Choose Add Video Track or Insert Track to add another track.
5. Click the mute button on V1.
6. The waveform on V1 doesn’t toggle on each click.

On a single-track mode, clicking the mute button toggles the waveform.

Moving around the clip on V1 causes erratic toggling effects of the mute bottoms. Here are some I was able to see:

If V2 is empty, mute is off on V2, and I move the clip from V1 to V2, the mute button on V2 toggles the waveform.
If V2 is empty, mute is on on V2, and I move the clip from V1 to V2, the mute button on V2 doesn’t toggle the waveform.

If V2 already has a clip, mute is off on V2, and I move the clip from V1 to V2, the mute button on V2 toggles the waveforms of both clips.
If V2 already has a clip, mute is on on V2, and I move the clip from V1 to V2, the mute button on V2 toggles the waveform of the preexisting clip but not that of the newly moved clip.

If I move the clip from V2 to V1 again and mute is off on V1, the mute button on V1 toggles the waveform.
If I move the clip from V2 to V1 again and mute is on on V1, the mute button on V1 doesn’t toggle the waveform.

Removing V2 doesn’t restore the toggling effect of the mute button on V1

Adding an audio track doesn’t cause this problem.

Additional notes:
a) Only tested in Windows 7 64bit.
b) This happens both in 18.08.14 and 18.03.06. I’m presuming that all the versions between and earlier versions have this problem.
c) This happens on both two PCs of mine with different CPUs.

[shotcut]

This is fixed for the next release v18.09.