feat(defaults): enable 'termguicolors' by default when supported by terminal

Enable 'termguicolors' automatically when Nvim can detect that truecolor
is supported by the host terminal.

If $COLORTERM is set to "truecolor" or "24bit", or the terminal's
terminfo entry contains capabilities for Tc, RGB, or setrgbf and
setrgbb, then we assume that the terminal supports truecolor. Otherwise,
the terminal is queried (using both XTGETTCAP and SGR + DECRQSS). If the
terminal's response to these queries (if any) indicates that it supports
truecolor, then 'termguicolors' is enabled.

1 files changed, 45 insertions, 2 deletions

diff --git a/src/nvim/tui/tui.c b/src/nvim/tui/tui.c
index c71eb633e9..d625c22c76 100644
--- a/src/nvim/tui/tui.c
+++ b/src/nvim/tui/tui.c
@@ -148,7 +148,8 @@ static bool cursor_style_enabled = false;
 # include "tui/tui.c.generated.h"
 #endif
 
-void tui_start(TUIData **tui_p, int *width, int *height, char **term)
+void tui_start(TUIData **tui_p, int *width, int *height, char **term, bool *rgb)
+  FUNC_ATTR_NONNULL_ALL
 {
   TUIData *tui = xcalloc(1, sizeof(TUIData));
   tui->is_starting = true;
@@ -177,6 +178,7 @@ void tui_start(TUIData **tui_p, int *width, int *height, char **term)
   *width = tui->width;
   *height = tui->height;
   *term = tui->term;
+  *rgb = tui->rgb;
 }
 
 void tui_set_key_encoding(TUIData *tui)
@@ -334,6 +336,9 @@ static void terminfo_start(TUIData *tui)
   int konsolev = konsolev_env ? (int)strtol(konsolev_env, NULL, 10)
                               : (konsole ? 1 : 0);
 
+  // truecolor support must be checked before patching/augmenting terminfo
+  tui->rgb = term_has_truecolor(tui, colorterm);
+
   patch_terminfo_bugs(tui, term, colorterm, vtev, konsolev, iterm_env, nsterm);
   augment_terminfo(tui, term, vtev, konsolev, iterm_env, nsterm);
   tui->can_change_scroll_region =
@@ -1439,7 +1444,7 @@ void tui_suspend(TUIData *tui)
     tui_mouse_on(tui);
   }
   stream_set_blocking(tui->input.in_fd, false);  // libuv expects this
-  ui_client_attach(tui->width, tui->height, tui->term);
+  ui_client_attach(tui->width, tui->height, tui->term, tui->rgb);
 #endif
 }
 
@@ -1752,6 +1757,44 @@ static int unibi_find_ext_bool(unibi_term *ut, const char *name)
   return -1;
 }
 
+/// Determine if the terminal supports truecolor or not:
+///
+/// 1. If $COLORTERM is "24bit" or "truecolor", return true
+/// 2. Else, check terminfo for Tc, RGB, setrgbf, or setrgbb capabilities. If
+///    found, return true
+/// 3. Else, return false
+static bool term_has_truecolor(TUIData *tui, const char *colorterm)
+{
+  // Check $COLORTERM
+  if (strequal(colorterm, "truecolor") || strequal(colorterm, "24bit")) {
+    return true;
+  }
+
+  // Check for Tc and RGB
+  for (size_t i = 0; i < unibi_count_ext_bool(tui->ut); i++) {
+    const char *n = unibi_get_ext_bool_name(tui->ut, i);
+    if (n && (!strcmp(n, "Tc") || !strcmp(n, "RGB"))) {
+      return true;
+    }
+  }
+
+  // Check for setrgbf and setrgbb
+  bool setrgbf = false;
+  bool setrgbb = false;
+  for (size_t i = 0; i < unibi_count_ext_str(tui->ut) && (!setrgbf || !setrgbb); i++) {
+    const char *n = unibi_get_ext_str_name(tui->ut, i);
+    if (n) {
+      if (!setrgbf && !strcmp(n, "setrgbf")) {
+        setrgbf = true;
+      } else if (!setrgbb && !strcmp(n, "setrgbb")) {
+        setrgbb = true;
+      }
+    }
+  }
+
+  return setrgbf && setrgbb;
+}
+
 /// Patches the terminfo records after loading from system or built-in db.
 /// Several entries in terminfo are known to be deficient or outright wrong;
 /// and several terminal emulators falsely announce incorrect terminal types.