I'm seeing several issues when editing links in the Visual editor.

On a Mac using Safari 12.0.1 or Chrome 70.0.3538.110, running Mac OS 10.14.1., accessing Jira Server 7.12.3.

1. Copy a URL like https://www.google.com to the clipboard from any text editor.
2. Go to a Jira issue and add a comment, using the Visual editor.
3. Type some text, select it, and type command-K. Nothing happens.
4. Instead, select External Link from the Link menu in the toolbar. The selected text becomes a wiki-style link like test, which seems wrong since we're in a visual editor.
5. Select the text after the | character, "http://example.com", and type command-V to paste; this should replace the example URL with the one we copied.
6. Instead of changing the text after the | character to the pasted text, it changes http://example.com to a blue underlined link to https://www.google.com,. So the resulting text looks like [test|http://example.com]. You get a visual-style link within a wiki-style link.

The workaround to the problem above is to use Option-Shift-Command-V when pasting the URL.

Or there's this:

1. Go to a Jira issue and add a comment, using the Visual editor.
2. Type a URL followed by a space. It becomes a blue underlined link.
3. Backspace over the end of the URL and type some text to change it. For example, change https://www.google.com to https://www.google.biz.
4. Type a space and continue typing. Everything you type becomes part of the URL, even after the space. So it ends up looking like: https://www.google.biz etc etc
5. In addition, the actual destination of the link is unchanged; it still goes to https://www.google.com.
6. And you can't edit the destination either. Selecting External Link from the toolbar just wraps another wiki-style link around the original one.