'guicursor': Disable by default for unknown terminals.

User can still set guicursor explicitly in init.vim.

Closes #5990
Closes #6403

runtime/doc/options.txt

@@ -2796,7 +2796,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 					-blinkwait175-blinkoff150-blinkon175")
 			global
 	Configures the cursor style for each mode. Works in the GUI and some
-	terminals. Empty means "non-blinking block cursor in all modes": >
+	terminals. Unset to disable: >
 		:set guicursor=
 <
 	With tmux you might need this in ~/.tmux.conf (see terminal-overrides

src/nvim/main.c

@@ -283,7 +283,7 @@ int main(int argc, char **argv)
   cmdline_row = (int)(Rows - p_ch);
   msg_row = cmdline_row;
   screenalloc(false);           /* allocate screen buffers */
-  set_init_2();
+  set_init_2(params.headless);
   TIME_MSG("inits 2");
 
   msg_scroll = TRUE;

src/nvim/option.c

@@ -939,11 +939,8 @@ void free_all_options(void)
 #endif
 
 
-/*
- * Initialize the options, part two: After getting Rows and Columns and
- * setting 'term'.
- */
-void set_init_2(void)
+/// Initialize the options, part two: After getting Rows and Columns.
+void set_init_2(bool headless)
 {
   int idx;
 
@@ -966,8 +963,12 @@ void set_init_2(void)
     p_window = Rows - 1;
   }
   set_number_default("window", Rows - 1);
-  parse_shape_opt(SHAPE_CURSOR);   /* set cursor shapes from 'guicursor' */
-  (void)parse_printoptions();       /* parse 'printoptions' default value */
+  if (!headless && !os_term_is_nice()) {
+    set_string_option_direct((char_u *)"guicursor", -1, (char_u *)"",
+                             OPT_GLOBAL, SID_NONE);
+  }
+  parse_shape_opt(SHAPE_CURSOR);   // set cursor shapes from 'guicursor'
+  (void)parse_printoptions();      // parse 'printoptions' default value
 }
 
 /*
@@ -2842,9 +2843,10 @@ did_set_string_option (
     }
   }
 
-  /* 'guicursor' */
-  else if (varp == &p_guicursor)
+  // 'guicursor'
+  else if (varp == &p_guicursor) {
     errmsg = parse_shape_opt(SHAPE_CURSOR);
+  }
 
   else if (varp == &p_popt)
     errmsg = parse_printoptions();

src/nvim/os/env.c

@@ -889,3 +889,17 @@ bool os_setenv_append_path(const char *fname)
   }
   return false;
 }
+
+/// Returns true if the terminal can be assumed to silently ignore unknown
+/// control codes.
+bool os_term_is_nice(void)
+{
+#if defined(__APPLE__) || defined(WIN32)
+  return true;
+#else
+  const char *vte_version = os_getenv("VTE_VERSION");
+  return (vte_version && atoi(vte_version) >= 3900)
+    || NULL != os_getenv("KONSOLE_PROFILE_NAME")
+    || NULL != os_getenv("KONSOLE_DBUS_SESSION");
+#endif
+}

test/functional/ui/cursor_spec.lua

@@ -3,6 +3,7 @@ local Screen = require('test.functional.ui.screen')
 local clear, meths = helpers.clear, helpers.meths
 local eq = helpers.eq
 local command = helpers.command
+local wait = helpers.wait
 
 describe('ui/cursor', function()
   local screen
@@ -18,7 +19,7 @@ describe('ui/cursor', function()
   end)
 
   it("'guicursor' is published as a UI event", function()
-    command('redraw')
+    wait()
     screen:expect('', nil, nil, nil, true)  -- Tickle the event-loop.
     local expected_cursor_style = {
       cmdline_hover = {
@@ -149,13 +150,13 @@ describe('ui/cursor', function()
 
     -- Event is published ONLY if the cursor style changed.
     screen._cursor_style = nil
-    command('redraw')
+    wait()
     screen:expect('', nil, nil, nil, true)  -- Tickle the event-loop.
     eq(nil, screen._cursor_style)
 
     -- Change the 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')
-    command('redraw')
+    wait()
     screen:expect('', nil, nil, nil, true)  -- Tickle the event-loop.
     eq('vertical', screen._cursor_style.normal.cursor_shape)
     eq('horizontal', screen._cursor_style.visual_select.cursor_shape)
@@ -171,6 +172,8 @@ describe('ui/cursor', function()
     meths.set_option('guicursor', '')
     command('redraw')
     screen:expect('', nil, nil, nil, true)  -- Tickle the event-loop.
+    -- Empty 'guicursor' sets enabled=false.
+    eq(false, screen._cursor_style_enabled)
     for _, m in ipairs({ 'cmdline_insert', 'cmdline_normal', 'cmdline_replace', 'insert',
                          'showmatch', 'normal', 'replace', 'visual',
                          'visual_select', }) do