eval: Add +tablineat feature

runtime/doc/eval.txt

@@ -7063,6 +7063,7 @@ statusline		Compiled with support for 'statusline', 'rulerformat'
 syntax			Compiled with syntax highlighting support |syntax|.
 syntax_items		There are active syntax highlighting items for the
 			current buffer.
+tablineat		'tabline' option accepts %@Func@ items.
 tag_binary		Compiled with binary searching in tags files
 			|tag-binary-search|.
 tag_old_static		Compiled with support for old static tags

runtime/doc/options.txt

@@ -6042,6 +6042,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	         is a bug that denotes that new mouse button recognition was 
 	         added without modifying code that reacts on mouse clicks on 
 	         this label.
+	      Note: to test whether your version of Neovim contains this 
+	            feature use `has('tablineat')`.
 	< -   Where to truncate line if too long.  Default is at the start.
 	      No width fields allowed.
 	= -   Separation point between left and right aligned items.

runtime/doc/various.txt

@@ -363,6 +363,7 @@ N  *+startuptime*	|--startuptime| argument
 N  *+statusline*	Options 'statusline', 'rulerformat' and special
 			formats of 'titlestring' and 'iconstring'
 N  *+syntax*		Syntax highlighting |syntax|
+N  *+tablineat*		'tabline' option recognizing %@Func@ items.
 N  *+tag_binary*	binary searching in tags file |tag-binary-search|
 N  *+tag_old_static*	old method for static tags |tag-old-static|
 m  *+tag_any_white*	any white space allowed in tags file |tag-any-white|

src/nvim/eval.c

@@ -10930,6 +10930,7 @@ static void f_has(typval_T *argvars, typval_T *rettv)
 #if !defined(UNIX)
     "system",  // TODO(SplinterOfChaos): This IS defined for UNIX!
 #endif
+    "tablineat",
     "tag_binary",
     "tag_old_static",
     "termresponse",

test/functional/ui/mouse_spec.lua

@@ -2,7 +2,7 @@ local helpers = require('test.functional.helpers')
 local Screen = require('test.functional.ui.screen')
 local clear, feed, meths = helpers.clear, helpers.feed, helpers.meths
 local insert, execute = helpers.insert, helpers.execute
-local eq = helpers.eq
+local eq, funcs = helpers.eq, helpers.funcs
 
 describe('Mouse input', function()
   local screen
@@ -167,6 +167,7 @@ describe('Mouse input', function()
       local test_click = function(name, click_str, click_num, mouse_button,
                                   modifiers)
         it(name .. ' works', function()
+          eq(1, funcs.has('tablineat'))
           feed(click_str .. '<3,0>')
           check_reply({0, click_num, mouse_button, modifiers})
           feed(click_str .. '<4,0>')