Now that comments and TOC can be closed at the same time, we use a flex
layout so the main content uses the available width.
We're also making the comments work better on medium-sized screens,
since previously they had a fixed width and now the width is adapted to
the size of the screen.
Since now the comment box element has a relative position instead of an
absolute one, we need to consider the draft panel height when
calculating the comment box position.
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