Thanks @labudis! My main concern is:

• until the same editor is rolled site-wide, incl. PR, reviews, commit comments etc. (or until actually my a11y issues below resolved) — I'd prefer to opt out to have all the authoring experience consistent between the different surfaces. However the new editor is not accompanied by a "preview" badge, with means to disable etc. incl. in one's feature preview settings.
• input accessibility outline is not outlining the actual form element, but an arbitrary section that's not all writeable, so while it may have a value aesthetically, it's a little puzzling affordance; also it makes the whole textarea styled "flat", not defined — i.e. compared to the classic editor it's not immediately visible it's an editable form element ready to start typing.
• my UA (Safari) allows me to adjust any size of textarea to my liking, and while I appreciate the input stretching with more content, the original behavior of stopping at a reasonable size; and/or not interfering with my custom sizing changes in browser afterwards, was superior to the new logic of stretching far beyond the space available, having "scrollbars within scrollbars", and constantly taking over the size even after my adjustments for my writing comforts.

(don't take it the wrong way: I'm not in the usual anti-change crowd; and I realize form element styling is hard in general, so this is just trying to point out some day-to-day usability kinks by examples, that just surprised me…)

BUG: The new issue authoring interface shows that CMD+enter submits the form; however the same is not shown on issue comments using the new editor, but still triggers — i.e. I just unexpectedly submitted an unfinished issue comment just by hitting that shortcut — there's no warning or clear affordance like it's in the new issue flow, but works the same way.

Makes sense:

This was unexpected to submit tho:

NIT: While I love to see the toolbar buttons for "References" (hash#) and "Mention" (handle@) gone to save space, the removal of "Attach files" to only keep it under the textarea among the action buttons is a little inconvenient — I'd prefer to have it where the muscle memory expects it, space allowing.

(EDIT: The issue is related to the above, stretching of the textarea taller than the viewport — if I then scroll back to reach the toolbar, it's inconvenient to again scroll down to reach the attachment button that's missing from the toolbar… when it's "below the fold" now;D)

BUG: The new issue authoring interface shows that CMD+enter submits the form; however the same is not shown on issue comments using the new editor, but still triggers — i.e. I just unexpectedly submitted an unfinished issue comment just by hitting that shortcut — there's no warning or clear affordance like it's in the new issue flow, but works the same way.

I've logged this as a bug - thanks for reporting!

Thanks for bringing this to our attention @janbrasna!