Composer buttons and selected text broken on Edge, IE11

[NedFodder]

When you highlight text and click on one of the formatting buttons (bold, italic, etc.), strange things happen (but only on Edge and IE11):

Different things happen depending on how many words are selected. I've reproduced the behavior here on community.nodebb.org. I'd report this on github, but I don't know which repo is responsible...

[pichalite]

@NedFodder you should report it here https://github.com/NodeBB/nodebb-plugin-composer-default