New to v5.9.0, Hover Expressions are action code expressions, like Display Expressions, but which (for Map view only) are displayed in a tooltip-like floating pane to the right of a map icon when the icon is moused over. This allows extra information to be viewed without having to open the note. A trivial example might be to show the word count for a note's $Text.
The expression is stored in attribute $HoverExpression and is a string of action code. It might be an attribute reference:
$WordCount
or a simple string:
"Don't forget to review this!"
or an expression:
"Word count: " + $WordCount
$Price * $NumUnits
Line breaks are allowed in strings using a the '\n' construct:
"Word count: " + $WordCount + "\n\n" +$Text
The expression's hover panel always draws test in $NameFont, with the font size controlled by $MapTextSize. The panel is always a translucent dark panel with white text. The panel is drawn 250 pixels wide with height never exceeding half the Map view's window height.
This feature isn't intended to preview very large amounts of $Text but rather to do things like expose extra contextual information - much in the way Key Attributes and Display Expressions are used.
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).