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Since the dropdown box can become a circular menu by pressing the space
bar on it, knowing that the mbrola voices are at the bottom of the list
makes finding them a bit easier using the space bar followed by the up
arrow key. Also, the end key, if your keyboard makes it available, will
take you to the last variant on the bottom of the list as well. I
recognize that this can make it harder to find a specific set of variant
voices such as espeak-ng Mbrola when you don't know that they're at the
bottom of the list, but it can help once you find it for the first time.
It's a bit like finding that link all the way at the bottom of a website
- a bit hard to find the first time, but much easier the second time.
Hope this helps.