Leaving the ground

So interestingly enough, the "leaving base without items" message will appear if you use a leap, which makes it an ideal way to test and see when exactly leaps and the like leave the ground. Or in some cases, go into the ground.
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For convenience's sake, I've sorted the leaps and similar abilities into a few categories based on their mechanics.
The images are the exact frame at which the game considers you to have "left the ground", which is to say, unable to be hit. Of course, this is done without any sort of lag, so in an actual game you're likely to be vulnerable for a few more frames.
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Standard Leap
http://imgur.com/a/WyJ4q
Of note: Serqet's takes a few extra frames than most to "leave the ground"
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Flight
http://imgur.com/a/TY3hH
Of note: Arachne and Freya are both considered still on the ground until the animation reaches maximum height.
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Teleport
http://imgur.com/a/bM18q
Of note: They do exactly what you think they do. Also, you're not "there" until 1 frame after you appear
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Banish (So technically every one of these is sort of a like a "mini-banish", but Janus's thing doesn't really fit in with the rest)
http://imgur.com/yRWbeKU
Of note: N/A
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Brutalize
This is a funny ability. Mechanically, it's a leap. You can jump over gods, minions, and some of the thinner walls with it. Actually, when you land on someone, it's rather like a short leap followed by 3 auto-targetting leaps that will jump you to wherever your target goes, provided they've not traveled too far i.e. with their own leap. That being said, in my testing I found that Brutalize doesn't actually cause you to leave the ground. Odd given that you can travel through walls with it, but moving along. Brutalize can be interrupted by CC, but most people tend to catch it on the leap, as opposed to during the travel time.
http://imgur.com/a/ZtQ1m
If you'll notice, Brutalize doesn't allow Fenrir to avoid the Sic 'em. However, instead of being rooted to where he and the dogs made "contact", Fenrir was instead rooted to where his destination was. This combined with the whole going-through-walls mechanic means it's not a dash, but is instead probably better categorized as the only leap in which you are vulnerable.
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King of the Eastern Seas
Due to a quirk of the ability, testing to see when the dragon left the ground was essentially impossible. When you successfully cast this ability, the period in which Ao moves to your target (in between your click and the actual stun of your target), Ao is essentially banished. This only lasts for an extremely short time however, and that 1s Ao stuns his target before Knockup/execute, he is completely vulnerable
http://imgur.com/a/GnugF
Notice how before casting Ao is affected by the enemy witchblade and mystical mail, and how those effects disappear until the stun actually begins.
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In conclusion: Remember there's latency and time your abilities better.