TUI: can make the cursor transparent #11519

when setting 'guicursor' highlight blend=100.

runtime/doc/term.txt

@@ -149,6 +149,8 @@ Nvim will adjust the shape of the cursor from a block to a line when in insert
 mode (or as specified by the 'guicursor' option), on terminals that support
 it.  It uses the same |terminfo| extensions that were pioneered by tmux for
 this: "Ss" and "Se".
+Similarly, if you set the cursor highlight group with blend=100, Nvim hides
+the cursor through the "cvvis" and "civis" extensions.
 
 If your terminfo definition is missing them, then Nvim will decide whether to
 add them to your terminfo definition, by looking at $TERM and other

runtime/doc/ui.txt

@@ -294,7 +294,8 @@ numerical highlight ids to the actual attributes.
 	`underline`:	underlined text. The line has `special` color.
 	`undercurl`:	undercurled text. The curl has `special` color.
 	`blend`:	Blend level (0-100). Could be used by UIs to support
-			blending floating windows to the background.
+			blending floating windows to the background or to
+			signal a transparent cursor.
 
 	For absent color keys the default color should be used. Don't store
 	the default value in the table, rather a sentinel value, so that

src/nvim/tui/tui.c

@@ -100,7 +100,7 @@ typedef struct {
   bool immediate_wrap_after_last_column;
   bool bce;
   bool mouse_enabled;
-  bool busy, is_invisible;
+  bool busy, is_invisible, want_invisible;
   bool cork, overflow;
   bool cursor_color_changed;
   bool is_starting;
@@ -198,6 +198,7 @@ static void terminfo_start(UI *ui)
   data->default_attr = false;
   data->can_clear_attr = false;
   data->is_invisible = true;
+  data->want_invisible = false;
   data->busy = false;
   data->cork = false;
   data->overflow = false;
@@ -1032,7 +1033,11 @@ static void tui_set_mode(UI *ui, ModeShape mode)
 
   if (c.id != 0 && c.id < (int)kv_size(data->attrs) && ui->rgb) {
     HlAttrs aep = kv_A(data->attrs, c.id);
-    if (aep.rgb_ae_attr & HL_INVERSE) {
+
+    data->want_invisible = aep.hl_blend == 100;
+    if (data->want_invisible) {
+      unibi_out(ui, unibi_cursor_invisible);
+    } else if (aep.rgb_ae_attr & HL_INVERSE) {
       // We interpret "inverse" as "default" (no termcode for "inverse"...).
       // Hopefully the user's default cursor color is inverse.
       unibi_out_ext(ui, data->unibi_ext.reset_cursor_color);
@@ -1980,10 +1985,12 @@ static void flush_buf(UI *ui)
     assert(data->is_invisible);
     // not busy and the cursor is invisible. Write a "cursor normal" command
     // after writing the buffer.
-    bufp->base = data->norm;
-    bufp->len = UV_BUF_LEN(data->normlen);
-    bufp++;
-    data->is_invisible = data->busy;
+    if (!data->want_invisible) {
+      bufp->base = data->norm;
+      bufp->len = UV_BUF_LEN(data->normlen);
+      bufp++;
+    }
+    data->is_invisible = false;
   }
 
   uv_write(&req, STRUCT_CAST(uv_stream_t, &data->output_handle),

test/functional/ui/cursor_spec.lua

@@ -245,6 +245,25 @@ describe('ui/cursor', function()
       eq('normal', screen.mode)
     end)
 
+    -- update the highlight again to hide cursor
+    helpers.command('hi Cursor blend=100')
+
+    for _, m in ipairs(expected_mode_info) do
+      if m.hl_id then
+          m.attr = {background = Screen.colors.Red, blend = 100}
+      end
+    end
+    screen:expect{grid=[[
+      ^                         |
+      ~                        |
+      ~                        |
+      ~                        |
+      test                     |
+    ]], condition=function()
+      eq(expected_mode_info, screen._mode_info)
+    end
+    }
+
     -- Another cursor style.
     meths.set_option('guicursor', 'n-v-c:ver35-blinkwait171-blinkoff172-blinkon173'
       ..',ve:hor35,o:ver50,i-ci:block,r-cr:hor90,sm:ver42')