At http://docs.webix.com/samples/60_pro/02_form/06_multicombo.html, if you type “full” then press Tab, the whole dropdown disappears. Same if you type “full”, then press down arrow to select “Full Metal Jacket”, then press Tab.
Better UX:
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Always select the first entry among those in the dropdown that match the input text. Pressing Tab will place the selected entry from the dropdown in the input. Up/Down arrows move selection among the entries in the dropdown. This is what Vanilla does and you can see it in the Tags input when you create a new discussion here in this forum. Bootstrap Typeahead and Pinboard.in use the same UX for tagging. I’ve used these for years and found them very effective. Select2 has an open issue to implement this as well.
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Focus on the first option that matches the input, just like StackOverflow does when you edit the tags on a question. I don’t favor this approach, but it’s still better than hiding the entire dropdown when pressing tab.