Merge pull request #8055 from bfredl/strictwritedelay

make 'writedelay' show all redraws when negative

runtime/doc/options.txt

@@ -6922,7 +6922,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 'writedelay' 'wd'	number	(default 0)
 			global
 	The number of milliseconds to wait for each character sent to the
-	screen.  When non-zero, characters are sent to the terminal one by
-	one.  For debugging purposes.
+	screen.  When positive, characters are sent to the UI one by one.
+	When negative, all redrawn characters cause a delay, even if the
+	character already was displayed by the UI.  For debugging purposes.
 
  vim:tw=78:ts=8:ft=help:noet:norl:

src/nvim/screen.c

@@ -4361,7 +4361,8 @@ static int char_needs_redraw(int off_from, int off_to, int cols)
                       && comp_char_differs(off_from, off_to))
                   || ((*mb_off2cells)(off_from, off_from + cols) > 1
                       && ScreenLines[off_from + 1]
-                      != ScreenLines[off_to + 1])))));
+                      != ScreenLines[off_to + 1])))
+          || p_wd < 0));
 }
 
 /*

src/nvim/ui.c

@@ -437,9 +437,8 @@ void ui_puts(uint8_t *str)
 
     if (p_wd) {  // 'writedelay': flush & delay each time.
       ui_flush();
-      assert(p_wd >= 0
-             && (sizeof(long) <= sizeof(uint64_t) || p_wd <= UINT64_MAX));
-      os_delay((uint64_t)p_wd, false);
+      uint64_t wd = (uint64_t)labs(p_wd);
+      os_delay(wd, false);
     }
   }
 }