When the user moves the pointer near the right margin of the document, a hovering "Add comment" button should be displayed. When the user clicks it, a new area selection and a corresponding new comment window should appear:
The new area selection should span whole page width, with a predefined height, and the comment number should be attached to its top right corner. The user should be able to modify size and position of area selection, except for the top left corner, which should be fixed.
The new comment window should be inserted between existing comments in the comment section, depending on the relative positions of the top left corner of the area selections of the new and existing comments, and should contain the comment number, the user avatar, the user display name, a text field entry, and Comment/Cancel buttons.
DISCUSS: How to place the new/saved window when there are too many existing comments/replies in the comments section already? (Will we use in-line scrolling or other solution?) Where to place the new window, and where to place after the user submits the comment?