Since Youtube Gallery 3.2.9

There is new tags added to Navigation Bar Custom Layout: [if:isactive] and [ifnot:isactive]

So to hide thumbnail if it is playing:

To highlight active thumbnail:

Note: it doesn't work with Hot Switch (when "Switch Video As" set to "Update Player without page reload") is not affiliated with or endorsed by the Joomla! Project or Open Source Matters.
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