UI: fix cursor not displayed after hiding and un-hiding #12811

- TUI: Fix a case where the cursor was not displayed after hiding the
  cursor and then setting it to be displayed again.
- Change to reset everything before setting guicursor.

fixes #12800
close #12811

Steps to reproduce:

    nvim -u NORC
    :set termguicolors
    :hi nCursor guifg=red guibg=red
    :hi iCursor guifg=green guibg=green
    :hi cCursor guifg=blue guibg=blue
    :set guicursor=n:block-nCursor,i:hor25-iCursor,c:ver25-cCursor
    :set guicursor-=c:ver25-cCursor

Actual behaviour: Cursor is a blue vertical.
Expected behaviour: Cursor should be the default color block.

1 files changed, 17 insertions, 0 deletions

diff --git a/test/functional/ui/cursor_spec.lua b/test/functional/ui/cursor_spec.lua
index 6c913124ac..e1a72ced05 100644
--- a/test/functional/ui/cursor_spec.lua
+++ b/test/functional/ui/cursor_spec.lua
@@ -286,6 +286,21 @@ describe('ui/cursor', function()
       eq(173, named.normal.blinkon)
       eq(42, named.showmatch.cell_percentage)
     end)
+
+    -- If there is no setting for guicursor, it becomes the default setting.
+    meths.set_option('guicursor', 'n:ver35-blinkwait171-blinkoff172-blinkon173-Cursor/lCursor')
+    screen:expect(function()
+      for _,m in ipairs(screen._mode_info) do
+        if m.name ~= 'normal' then
+          eq('block', m.cursor_shape or 'block')
+          eq(0, m.blinkon or 0)
+          eq(0, m.blinkoff or 0)
+          eq(0, m.blinkwait or 0)
+          eq(0, m.hl_id or 0)
+          eq(0, m.id_lm or 0)
+        end
+      end
+    end)
   end)
 
   it("empty 'guicursor' sets cursor_shape=block in all modes", function()
@@ -297,6 +312,8 @@ describe('ui/cursor', function()
         if m['cursor_shape'] ~= nil then
           eq('block', m.cursor_shape)
           eq(0, m.blinkon)
+          eq(0, m.hl_id)
+          eq(0, m.id_lm)
         end
       end
     end)