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.

runtime/doc/options.txt

@@ -2738,21 +2738,26 @@ A jump table for the options with a short description can be found at |Q_op|.
 		hor{N}	horizontal bar, {N} percent of the character height
 		ver{N}	vertical bar, {N} percent of the character width
 		block	block cursor, fills the whole character
-			[only one of the above three should be present]
+			- Only one of the above three should be present.
+			- Default is "block" for each mode.
 		blinkwait{N}				*cursor-blinking*
 		blinkon{N}
 		blinkoff{N}
 			blink times for cursor: blinkwait is the delay before
 			the cursor starts blinking, blinkon is the time that
 			the cursor is shown and blinkoff is the time that the
-			cursor is not shown.  The times are in msec.  When one
+			cursor is not shown.  Times are in msec.  When one of
-			of the numbers is zero, there is no blinking. E.g.: >
+			the numbers is zero, there is no blinking. E.g.: >
 				:set guicursor=n:blinkon0
-<		{group-name}
+<			- Default is "blinkon0" for each mode.
-			Highlight group name that sets the color and font for
+		{group-name}
-			the cursor. |inverse|/reverse and no group-name are
+			Highlight group that decides the color and font of the
-			interpreted as "the host terminal default cursor
+			cursor.
-			colors" which usually invert bg and fg colors.
+			In the |TUI|:
+			- |inverse|/reverse and no group-name are interpreted
+			  as "host-terminal default cursor colors" which
+			  typically means "inverted bg and fg colors".
+			- |ctermfg| and |guifg| are ignored.
 		{group-name}/{group-name}
 			Two highlight group names, the first is used when
 			no language mappings are used, the other when they

src/nvim/cursor_shape.c

@@ -13,6 +13,10 @@
 #include "nvim/api/private/helpers.h"
 #include "nvim/ui.h"
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "cursor_shape.c.generated.h"
+#endif
+
 /// Handling of cursor and mouse pointer shapes in various modes.
 cursorentry_T shape_table[SHAPE_IDX_COUNT] =
 {
@@ -77,7 +81,9 @@ Array mode_style_array(void)
   return all;
 }
 
-/// Parse the 'guicursor' option
+/// Parses the 'guicursor' option.
+///
+/// Clears `shape_table` if 'guicursor' is empty.
 ///
 /// @param what SHAPE_CURSOR or SHAPE_MOUSE ('mouseshape')
 ///
@@ -99,11 +105,17 @@ char_u *parse_shape_opt(int what)
 
   // First round: check for errors; second round: do it for real.
   for (round = 1; round <= 2; round++) {
+    if (round == 2 || *p_guicursor == NUL) {
+      // Set all entries to default (block, blinkon0, default color).
+      // This is the default for anything that is not set.
+      clear_shape_table();
+      if (*p_guicursor == NUL) {
+        ui_mode_info_set();
+        return NULL;
+      }
+    }
     // Repeat for all comma separated parts.
     modep = p_guicursor;
-    if (*p_guicursor == NUL) {
-      modep = (char_u *)"a:block-blinkon0";
-    }
     while (modep != NULL && *modep != NUL) {
       colonp = vim_strchr(modep, ':');
       commap = vim_strchr(modep, ',');
@@ -144,14 +156,6 @@ char_u *parse_shape_opt(int what)
 
         if (all_idx >= 0) {
           idx = all_idx--;
-        } else if (round == 2) {
-          {
-            // Set the defaults, for the missing parts
-            shape_table[idx].shape = SHAPE_BLOCK;
-            shape_table[idx].blinkwait = 0L;
-            shape_table[idx].blinkon = 0L;
-            shape_table[idx].blinkoff = 0L;
-          }
         }
 
         /* Parse the part after the colon */
@@ -330,3 +334,16 @@ int cursor_get_mode_idx(void)
     return SHAPE_IDX_N;
   }
 }
+
+/// Clears all entries in shape_table to block, blinkon0, and default color.
+static void clear_shape_table(void)
+{
+  for (int idx = 0; idx < SHAPE_IDX_COUNT; idx++) {
+    shape_table[idx].shape = SHAPE_BLOCK;
+    shape_table[idx].blinkwait = 0L;
+    shape_table[idx].blinkon = 0L;
+    shape_table[idx].blinkoff = 0L;
+    shape_table[idx].id = 0;
+    shape_table[idx].id_lm = 0;
+  }
+}

src/nvim/tui/tui.c

@@ -1083,6 +1083,7 @@ static void tui_set_mode(UI *ui, ModeShape mode)
     }
   } else if (c.id == 0) {
     // No cursor color for this mode; reset to default.
+    data->want_invisible = false;
     unibi_out_ext(ui, data->unibi_ext.reset_cursor_color);
   }
 

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)