This issue is limited to the latest version of Desktop Chrome (v77, as of writing), it's not a change on our part, we think it's a change to Chrome's layout rendering engine.
Steps to replicate
Log in and go to your inbox
Make sure you have some comments in your Inbox
Click the "Reply" button to a comment
The floating comment box appears, click into the textarea
The box disappears from view, and will reappear depending on where you move your cursor, but trying to interact with any of the form fields will make it disappear again.
You will find the comment box when you scroll all the way to the bottom of the page, that's where it will stay even if you interact with it.
When you click "Reply" to make the box first show up, and click into the textarea, the box disappears, but you have put focus on the textarea. If you start typing, even though you don't see the box, the browser will jump to the bottom of the page to the textarea so that you can see what you're typing.
When you click "Reply" to make the box first show up, then drag the box, that will ensure it stays put.
What should happen
The box should stay at the position where it was first initiated, even when focusing form elements inside of it.
One current theory is that this jumping happens because the box is created inside a container that is absolute positioned but doesn't have a top/right/bottom/left value applied. Once you set a top/left value by dragging the box, it will stay positioned there.
The floating box is used in two other places, the issue doesn't occur there. For the bookmarks form, because the top/left value is overwritten. For the comment box on the "Unread Messages" section on the homepage, because it gets rendered at the bottom of the container anyway (aka the place where it would jump to).