The forum editor is not only wonky in IE 11, but also not fully compatible with any other web browser and apparently a known problem affecting also other community sites relying on Jive. I wonder what happened to the software update that was supposed to happen in November. If there was any, I have not noticed any editor improvements worth mentioning so far. For cut and paste to work you may have to use a different browser. Alternatively, the following should work:
Enable F12 Developer Tools from the Tools menu.
Change the default document mode from Edge to IE 10.
Select Leave this page.
Yes, I've experienced the joy of trying to format a response in the "editor" using other browsers too. It's, shall we say, an adventure...
One can also set the user-agent string as a workaround in place of changing the document mode - I was just surprised that the ability to do something as simple as paste (either via ctrl-p or right-click, select paste on context menu) into the editor became problematic. Perhaps I was just being naïve!
I accidentally upgraded to IE 11 some time ago, and I've found the biggest problem is that back-space will randomly delete a couple of lines in whatever I'm typing (often something near the beginning). The latest version of Chrome doesn't seem to have the same problem - but I keep forgetting to start up Chrome for OTN, so I often end up just not bothering to complete a response.
If I really want to persevere, the safest bet seems to be to go to the HTML option and type the html codes in manually.
According to many troubles being reported in Google related to cut and paste and other features, it seems some of the new security concepts of IE 11 are not compatible with various sites, in particular forum software. Features are often enabled based on the client browser type and version.
I'm glad I don't have to use MS Windows or IE. I think it has become very complex and over-engineered over the years and is hardly feasible anymore - actually never was as there were always better alternatives. However, it seems to be the trend also with other systems and software and it does not help if the UI simply gets dumbed down. Anyway, IE 11 is apparently not an option until the forum gets patched or updated.