Outlines, from v5.10.0, can display a note's $HoverExpression as the cursor hovers over a note (including separators).
If a note has no $HoverExpression (or its expression produces none), the note's Subtitle - if one - is displayed. Thus the subtitle can only be seen if there is no hover text.
If neither $HoverExpression nor $Subtitle are set, no hover pane is displayed during rollover of an outline item.
From v5.10.2, the hover expression box uses rounded corners and is suppressed if the expression only returns white space (i.e. no visible text); also, the box is offset further right so as not to obscure $Name data.