fix(gui): use class selectors for sidebar-scrollbox in GUI tests

[jhult]

Replace bare mdbook-sidebar-scrollbox tag-name selectors with .sidebar-scrollbox class selectors in GUI tests to avoid timing-dependent failures when the custom element hasn't been registered yet in headless Chrome.

[jhult]

Found via failure on #3049

[ehuss]

Thanks! Do you have more information on why the tag-name would be any different than the class? I would expect that the DOM is ready when the test runs, so I would not expect there to be any difference. Do you have a testing method to verify this is the right fix?

cc @GuillaumeGomez in case you have any knowledge on this.

[GuillaumeGomez]

I'm also curious about the reason (for personal knowledge). Other than that, changes look good to me.

[ehuss]

Hm, I did a little digging. I can't find a clear source, but it sounds like there is no guarantee that the customElement constructor is finished before DOMContentLoaded. There is a separate whenDefined when the definition is complete, but that wouldn't really be practical here.

[GuillaumeGomez]

To be noted: instead of assert-text, wait-for-text could be used.

[ehuss]

Thanks, I'm going to go ahead and merge since it seems to be more reliable in my testing.

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