>>1025364

Your example doesn't even work in my browsers (Chromium 46 and Firefox 33). Had to change `let` into `var` to fix the  problem. Chromium reports the following error: `Uncaught SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode`

And by the way display:flex is not so widely supported yet and has lot of issues: https://caniuse.com/#search=flex

The reason of delay os that max-height is animated from 0 to 900px and from 900px to 0. As your menu height is less than 900px you see no changes until max-height becomes less than menu height.

Please look at this example to see how it works: https://codepen.io/anon/pen/owVRVY

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Why use English I wonder?