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, 5 insertions, 4 deletions

diff --git a/src/nvim/ui_client.c b/src/nvim/ui_client.c
index eb32c16881..d744560a86 100644
--- a/src/nvim/ui_client.c
+++ b/src/nvim/ui_client.c
@@ -70,14 +70,14 @@ uint64_t ui_client_start_server(int argc, char **argv)
   return channel->id;
 }
 
-void ui_client_attach(int width, int height, char *term)
+void ui_client_attach(int width, int height, char *term, bool rgb)
 {
   MAXSIZE_TEMP_ARRAY(args, 3);
   ADD_C(args, INTEGER_OBJ(width));
   ADD_C(args, INTEGER_OBJ(height));
 
   MAXSIZE_TEMP_DICT(opts, 9);
-  PUT_C(opts, "rgb", BOOLEAN_OBJ(true));
+  PUT_C(opts, "rgb", BOOLEAN_OBJ(rgb));
   PUT_C(opts, "ext_linegrid", BOOLEAN_OBJ(true));
   PUT_C(opts, "ext_termcolors", BOOLEAN_OBJ(true));
   if (term) {
@@ -111,9 +111,10 @@ void ui_client_run(bool remote_ui)
   ui_client_is_remote = remote_ui;
   int width, height;
   char *term;
-  tui_start(&tui, &width, &height, &term);
+  bool rgb;
+  tui_start(&tui, &width, &height, &term, &rgb);
 
-  ui_client_attach(width, height, term);
+  ui_client_attach(width, height, term, rgb);
 
   // os_exit() will be invoked when the client channel detaches
   while (true) {