main: Fix color schemes for abstract_ui

- Set 't_Co' to 256 at startup. The value can be changed by the user for
  compatibility with terminals that are less capable.
- `has('gui_running')` will return 1 if at least one rgb UI is attached.

Even though these changes are hacky, they are necessary to make the transition
to the new UI architecture smoother.

src/nvim/eval.c

@@ -76,6 +76,7 @@
 #include "nvim/tag.h"
 #include "nvim/tempfile.h"
 #include "nvim/term.h"
+#include "nvim/ui.h"
 #include "nvim/mouse.h"
 #include "nvim/undo.h"
 #include "nvim/version.h"
@@ -10023,6 +10024,8 @@ static void f_has(typval_T *argvars, typval_T *rettv)
 #endif
     } else if (STRICMP(name, "syntax_items") == 0) {
       n = syntax_present(curwin);
+    } else if (STRICMP(name, "gui_running") == 0) {
+      n = ui_rgb_attached();
     }
   }
 
@@ -14440,7 +14443,7 @@ static void f_synIDattr(typval_T *argvars, typval_T *rettv)
     if (modec != 't' && modec != 'c' && modec != 'g')
       modec = 0;        /* replace invalid with current */
   } else {
-    if (t_colors > 1)
+    if (abstract_ui || t_colors > 1)
       modec = 'c';
     else
       modec = 't';

src/nvim/main.c

@@ -261,10 +261,7 @@ int main(int argc, char **argv)
 
   if (params.want_full_screen && !silent_mode) {
     if (embedded_mode) {
-      // In embedded mode don't do terminal-related initializations, assume an
-      // initial screen size of 80x20
-      full_screen = true;
-      screen_resize(80, 20, false);
+      // embedded mode implies abstract_ui
       termcapinit((uint8_t *)"abstract_ui");
     } else {
       // set terminal name and get terminal capabilities (will set full_screen)
@@ -278,7 +275,9 @@ int main(int argc, char **argv)
   event_init();
 
   if (abstract_ui) {
+    full_screen = true;
     t_colors = 256;
+    T_CCO = (uint8_t *)"256";
   } else {
     // Print a warning if stdout is not a terminal TODO(tarruda): Remove this
     // check once the new terminal UI is implemented

src/nvim/screen.c

@@ -6150,8 +6150,7 @@ void screen_fill(int start_row, int end_row, int start_col, int end_col, int c1,
     return;
 
   /* it's a "normal" terminal when not in a GUI or cterm */
-  norm_term = (
-    t_colors <= 1);
+  norm_term = (!abstract_ui && t_colors <= 1);
   for (row = start_row; row < end_row; ++row) {
     if (has_mbyte
         ) {
@@ -6675,8 +6674,8 @@ static void linecopy(int to, int from, win_T *wp)
  */
 int can_clear(char_u *p)
 {
-  return *p != NUL && (t_colors <= 1
-                       || cterm_normal_bg_color == 0 || *T_UT != NUL);
+  return abstract_ui || (*p != NUL && (t_colors <= 1
+        || cterm_normal_bg_color == 0 || *T_UT != NUL));
 }
 
 /*

src/nvim/syntax.c

@@ -6450,7 +6450,7 @@ do_highlight (
                   p = T_CAF;
                 else
                   p = T_CSF;
-                if (*p != NUL && *(p + STRLEN(p) - 1) == 'm')
+                if (abstract_ui || (*p != NUL && *(p + STRLEN(p) - 1) == 'm'))
                   switch (t_colors) {
                   case 16:
                     color = color_numbers_8[i];
@@ -6865,7 +6865,7 @@ int hl_combine_attr(int char_attr, int prim_attr)
   if (char_attr <= HL_ALL && prim_attr <= HL_ALL)
     return char_attr | prim_attr;
 
-  if (t_colors > 1) {
+  if (abstract_ui || t_colors > 1) {
     if (char_attr > HL_ALL)
       char_aep = syn_cterm_attr2entry(char_attr);
     if (char_aep != NULL)
@@ -6929,7 +6929,7 @@ int syn_attr2attr(int attr)
 {
   attrentry_T *aep;
 
-  if (t_colors > 1)
+  if (abstract_ui || t_colors > 1)
     aep = syn_cterm_attr2entry(attr);
   else
     aep = syn_term_attr2entry(attr);
@@ -7357,7 +7357,7 @@ int syn_id2attr(int hl_id)
   hl_id = syn_get_final_id(hl_id);
   sgp = &HL_TABLE()[hl_id - 1];             /* index is ID minus one */
 
-  if (t_colors > 1)
+  if (abstract_ui || t_colors > 1)
     attr = sgp->sg_cterm_attr;
   else
     attr = sgp->sg_term_attr;

src/nvim/ui.c

@@ -114,6 +114,16 @@ void ui_write(uint8_t *s, int len)
     free(tofree);
 }
 
+bool ui_rgb_attached(void)
+{
+  for (size_t i = 0; i < ui_count; i++) {
+    if (uis[i]->rgb) {
+      return true;
+    }
+  }
+  return false;
+}
+
 /*
  * If the machine has job control, use it to suspend the program,
  * otherwise fake it by starting a new shell.