Arrow keys get "caught" on triple-backquote language selection menu, impeding browsing through a document

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io

### What's the problem

Pressing/holding "down" to scroll through a document gets "caught" by triple-backquote language specification menu, and stops scrolling.

#### Current behaviour

Provide steps to reproduce:

1. Go to https://hackmd.io/IukON8-rRxadVtnx-Osxsw
2. Move the cursor to the end of the long heading line
3. Press or hold the down key to move down through the document.
4. When the cursor reaches the code block, observe the menu pop up to offer a selection of code block languages. This "catches" the cursor such that it can't escape without hitting the escape key.

#### Expected behaviour

This menu should not open automatically when navigating the document.

#### Environment

I use HackMD on:

Firefox 91 on Linux.