How to expand an UltiSnips snippet using <c-y> in the YouCompleteMe pop-up menu?

This problem really hit a nerve with me. I have both YouCompleteMe and UltiSnips installed on my vim 8.0 editor. It seems that both of these plugins use the tab key for doing the auto-completion and that has created an incompatibility that has been also addressed by this question. My question is more specific, though. When I write a piece of code like <html, there is a pop-up menu that shows me all related snippets for that code.

I use the tab key to navigate through that menu but when I hit ctrl+y to accept and therefore expand one of these snippets, nothing happens! I think this structure suggests that it's possible to somehow choose one of those snippets from the menu without trying to define a shortcut for UltiSnip. What am I doing wrong? How should I navigate and choose those snippets?

I also would not want to stuff my vim with any new plugins (like supertab, etc.).

The solution was actually a lot simpler than I expected. In the beginning, I felt stupid for not knowing it but when I find a similar question like this one, I thought that probably many were fallen into the same trap.

I don't know whether to name it a bug or not but it's how Ultisnips and YouCompleteMe work together. In order to expand a snippet, you have to write the initializer exactly as it's defined. Of course, this seems obvious, but when you see a pop-up menu of different snippets, you might think they can be chosen but it only works if you already wrote the snippet initializer exactly as it's defined.

So when a snippet is called "html5"---as it's shown in my question---writing an extra opening bracket (<) will cause it to stop working. It cannot be expanded.

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Also, don't forget to check out Siegfried Gevatter configuration. It's not possible to use tab key both for navigating into the pop-up menu and expanding the snippets.

P.S. It was nice if navigating through the pop-up menu could change the whole word (including the angle bracket), not just what succeeds it. This feature works this way in most of the other editors I see and that's probably why I wasn't able to spot the problem in the beginning.