Tab group's panel disappears after switching children tab group's panel

[gradepuxi22]

Describe the bug
Let's say we have tab group A and tab group B. Tab group B is inside A's tab panel. Switching panel in B makes A's tab panel disappear. After going inside the same A's tab panel through a tab, we can see that the panel in B has switched correctly.

To Reproduce
Steps to reproduce the behavior:

1. Create sl-tab-group with appropriate sl-tab and sl-tab-panel components,
2. Put another sl-tab-group with it's children inside previous' group sl-tab-panel,
3. While on the page, change tabs in the nested sl-tab-group.
4. Outer's sl-tab-group panels disappear and an error appears in the console:

Uncaught TypeError: Cannot read property 'clientWidth' of undefined
    at b.syncActiveTabIndicator (p-3c2cff2d.entry.js:1)
    at b.setActiveTab (p-3c2cff2d.entry.js:1)
    at b.handleClick (p-3c2cff2d.entry.js:1)

Expected behavior
Panels switch in the nested sl-tab-group without making the whole panel disappear.

Screenshots
Before clicking the tab:

After clicking the tab:

Error message:

Desktop (please complete the following information):

• OS: Ubuntu 20.04.1 LTS
• Browser Chrome
• Version 87.0.4280.88

[claviska]

Thanks for reporting this. Verified and fixed in a0e9297. I've also added a test to prevent regressions.