refactor(ui): cleanup 'redrawdebug', introduce "flush" mode

src/nvim/option_defs.h

@@ -630,6 +630,8 @@ EXTERN unsigned rdb_flags;
 #define RDB_NOTHROTTLE         0x002
 #define RDB_INVALID            0x004
 #define RDB_NODELTA            0x008
+#define RDB_LINE               0x010
+#define RDB_FLUSH              0x020
 
 EXTERN long p_rdt;            // 'redrawtime'
 EXTERN long p_re;             // 'regexpengine'

src/nvim/optionstr.c

@@ -125,7 +125,8 @@ static char *(p_spo_values[]) = { "camel", "noplainbuffer", NULL };
 static char *(p_icm_values[]) = { "nosplit", "split", NULL };
 static char *(p_jop_values[]) = { "stack", "view", NULL };
 static char *(p_tpf_values[]) = { "BS", "HT", "FF", "ESC", "DEL", "C0", "C1", NULL };
-static char *(p_rdb_values[]) = { "compositor", "nothrottle", "invalid", "nodelta", NULL };
+static char *(p_rdb_values[]) = { "compositor", "nothrottle", "invalid", "nodelta", "line",
+                                  "flush", NULL };
 static char *(p_sloc_values[]) = { "last", "statusline", "tabline", NULL };
 
 /// All possible flags for 'shm'.

src/nvim/ui.c

@@ -419,7 +419,7 @@ void ui_line(ScreenGrid *grid, int row, int startcol, int endcol, int clearcol,
                    (const sattr_T *)grid->attrs + off);
 
   // 'writedelay': flush & delay each time.
-  if (p_wd && !(rdb_flags & RDB_COMPOSITOR)) {
+  if (p_wd && (rdb_flags & RDB_LINE)) {
     // If 'writedelay' is active, set the cursor to indicate what was drawn.
     ui_call_grid_cursor_goto(grid->handle, row,
                              MIN(clearcol, (int)grid->cols - 1));
@@ -510,6 +510,10 @@ void ui_flush(void)
     pending_has_mouse = has_mouse;
   }
   ui_call_flush();
+
+  if (p_wd && (rdb_flags & RDB_FLUSH)) {
+    os_microdelay((uint64_t)labs(p_wd) * 1000U, true);
+  }
 }
 
 /// Check if 'mouse' is active for the current mode