Visual: highlight char-at-cursor

Decide whether to highlight the visual-selected character under the
cursor, depending on 'guicursor' style:

- Highlight if cursor is blinking or non-block (vertical, horiz).
- Do NOT highlight if cursor is non-blinking block.

Traditionally Vim's visual selection does "reverse mode", which perhaps
conflicts with the non-blinking block cursor. But 'guicursor' defaults
to a vertical bar for selection=exclusive, and this confuses users who
expect to see the text highlighted.

closes #8983

test/functional/ui/highlight_spec.lua

@@ -322,6 +322,40 @@ describe('highlight', function()
     screen:attach()
   end)
 
+  it('visual', function()
+    screen:detach()
+    screen = Screen.new(20,4)
+    screen:attach()
+    screen:set_default_attr_ids({
+      [1] = {background = Screen.colors.LightGrey},
+      [2] = {bold = true, foreground = Screen.colors.Blue1},
+      [3] = {bold = true},
+    })
+    insert([[
+      line1 foo bar
+    ]])
+
+    -- Non-blinking block cursor: does NOT highlight char-at-cursor.
+    command('set guicursor=a:block-blinkon0')
+    feed('gg$vhhh')
+    screen:expect([[
+        line1 foo^ {1:bar}     |
+                          |
+      {2:~                   }|
+      {3:-- VISUAL --}        |
+    ]])
+
+    -- Vertical cursor: highlights char-at-cursor. #8983
+    command('set guicursor=a:block-blinkon175')
+    feed('<esc>gg$vhhh')
+    screen:expect([[
+        line1 foo{1:^ bar}     |
+                          |
+      {2:~                   }|
+      {3:-- VISUAL --}        |
+    ]])
+  end)
+
   it('cterm=standout gui=standout', function()
     screen:detach()
     screen = Screen.new(20,5)