Get symbol at mouse position

Question

How can I get the symbol which is under the mouse pointer at any time?

I know that I can get the mouse position with MousePosition[] and dynamically with Dynamic[MousePosition[]]. However I was not able to find a way to get the symbol at this position.

My final goal is to create a second notebook which always shows the current value of the symbol which is under the mouse pointer.

Answer 1

Here's an attempt at something like this:

Dynamic@Row@{$sym, " = ", Symbol@$sym}

Unprotect@FE`CacheTemplateAndUsage;
FE`CacheTemplateAndUsage[sym_String] /; Context@arg == "Global`" :=
 (
  $sym = sym;
  MathLink`CallFrontEnd@
   FrontEnd`CacheTemplateAndUsagePacket[FrontEnd`InputNotebook[], sym,
     Null]
  )

The symbol name and its value is displayed in the top cell after hovering over a symbol for a few seconds. Some notes:

• This is using undocumented functionality, so use it at your own risk (it might also break at any moment)
• The idea is to intercept the call the front-end makes to get the usage information for a symbol. After this is successful once, it will never be done again, so we need to make sure to always return an empty usage
• Currently it is limited to symbols in the global context, since for system symbols you probably want to see the info dropdown
• The front-end needs to think that a value is assigned to the symbol (or something like that, not quite sure on the specifics). Usually any symbol with a definition should fall into this category, but in some circumstances the front-end is not aware of existing symbol definitions.