Use a div with attribute:
 - contenteditable="true"

It fixes a lot of issues:
 - height of #input needs no javascript to adapt to content
 - therefor no complex styles are needed
 - width will no longer exceed the given width by grid-template-column
 - styling etc. can be handled like the #output div next to it

You have to set two attributes for a11y:
 - role="textbox"
 - aria-multiline="true"

Signed-off-by: Conrad Zelck <git@simpel.cc>