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

runtime/doc/options.txt

@@ -4765,7 +4765,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Flags to change the way redrawing works, for debugging purposes.
 	Most useful with 'writedelay' set to some reasonable value.
 	Supports the following flags:
-	    compositor	Indicate what redraws come from the compositor
+	    compositor	Indicate each redraw event handled by the compositor
 			by briefly flashing the redrawn regions in colors
 			indicating the redraw type. These are the highlight
 			groups used (and their default colors):
@@ -4777,6 +4777,11 @@ A jump table for the options with a short description can be found at |Q_op|.
 		RedrawDebugRecompose guibg=Red    redraw generated by the
 						  compositor itself, due to a
 						  grid being moved or deleted.
+	    line	introduce a delay after each line drawn on the screen.
+			When using the TUI or another single-grid UI, "compositor"
+			gives more information and should be preferred (every
+			line is processed as a separate event by the compositor)
+	    flush	introduce a delay after each "flush" event.
 	    nothrottle	Turn off throttling of the message grid. This is an
 			optimization that joins many small scrolls to one
 			larger scroll when drawing the message area (with
@@ -7372,8 +7377,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 						*'writedelay'* *'wd'*
 'writedelay' 'wd'	number	(default 0)
 			global
-	The number of milliseconds to wait for each character sent to the
+	Only takes effect toghether with 'redrawdebug'.
-	screen.  When positive, characters are sent to the UI one by one.
+	The number of milliseconds to wait after each line or each flush
-	See 'redrawdebug' for more options.  For debugging purposes.
 
  vim:tw=78:ts=8:noet:ft=help:norl:

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