screen: don't crash on invalid grid cells being recomposed

runtime/doc/options.txt

@@ -4583,6 +4583,14 @@ 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.
+	    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
+			'display' msgsep flag active).
+	    invalid	Enable stricter checking (abort) of inconsistencies
+			of the internal screen state. This is mosly
+			useful when running nvim inside a debugger (and
+			the test suite).
 
 						*'redrawtime'* *'rdt'*
 'redrawtime' 'rdt'	number	(default 2000)

src/nvim/option_defs.h

@@ -518,10 +518,11 @@ EXTERN long p_pyx;              // 'pyxversion'
 EXTERN char_u *p_rdb;           // 'redrawdebug'
 EXTERN unsigned rdb_flags;
 # ifdef IN_OPTION_C
-static char *(p_rdb_values[]) = { "compositor", "nothrottle", NULL };
+static char *(p_rdb_values[]) = { "compositor", "nothrottle", "invalid", NULL };
 # endif
 # define RDB_COMPOSITOR         0x001
 # define RDB_NOTHROTTLE         0x002
+# define RDB_INVALID            0x004
 
 EXTERN long p_rdt;              // 'redrawtime'
 EXTERN int p_remap;             // 'remap'

src/nvim/ui_compositor.c

@@ -425,6 +425,15 @@ static void compose_line(Integer row, Integer startcol, Integer endcol,
     flags = flags & ~kLineFlagWrap;
   }
 
+  for (int i = skipstart; i < (endcol-skipend)-startcol; i++) {
+    if (attrbuf[i] < 0) {
+      if (rdb_flags & RDB_INVALID) {
+        abort();
+      } else {
+        attrbuf[i] = 0;
+      }
+    }
+  }
   ui_composed_call_raw_line(1, row, startcol+skipstart,
                             endcol-skipend, endcol-skipend, 0, flags,
                             (const schar_T *)linebuf+skipstart,
@@ -535,6 +544,11 @@ static void ui_comp_raw_line(UI *ui, Integer grid, Integer row,
   } else {
     compose_debug(row, row+1, startcol, endcol, dbghl_normal, false);
     compose_debug(row, row+1, endcol, clearcol, dbghl_clear, true);
+#ifndef NDEBUG
+    for (int i = 0; i < endcol-startcol; i++) {
+      assert(attrs[i] >= 0);
+    }
+#endif
     ui_composed_call_raw_line(1, row, startcol, endcol, clearcol, clearattr,
                               flags, chunk, attrs);
   }

test/functional/helpers.lua

@@ -38,7 +38,7 @@ module.nvim_prog = (
 module.nvim_set = (
   'set shortmess+=IS background=light noswapfile noautoindent'
   ..' laststatus=1 undodir=. directory=. viewdir=. backupdir=.'
-  ..' belloff= wildoptions-=pum noshowcmd noruler nomore')
+  ..' belloff= wildoptions-=pum noshowcmd noruler nomore redrawdebug=invalid')
 module.nvim_argv = {
   module.nvim_prog, '-u', 'NONE', '-i', 'NONE',
   '--cmd', module.nvim_set, '--embed'}