A bug report from Pamputt:
Users can't middle-click to paste text in CodeMirror.
Quiddity partially verifies:
I cannot middle-click paste in the normal wikitext editor if CodeMirror is active, with Firefox+Ubuntu (Mint).
But I can middle-click paste in VE, NWE (even with CodeMirror active), and the normal wikitext editor, with Firefox+Ubuntu.
Firefox 55 on Linux. Can confirm Pamputt's statement: With the Visual Editor , the text is not pasted at the correct position (but at the position the cursor is sitting in). With CodeMirror, it is not pasted at all.
I work a lot with Middle click – often I use it the be able to have two things in Copy-Paste at the same time.