tweaks: add a warning for a condition that should never occur

src/winio.c

@@ -2667,8 +2667,11 @@ int update_line(filestruct *fileptr, size_t index)
     row = fileptr->lineno - openfile->edittop->lineno;
 
     /* If the line is offscreen, don't even try to display it. */
-    if (row < 0 || row >= editwinrows)
+    if (row < 0 || row >= editwinrows) {
+	statusline(ALERT, "Badness: tried to display a chunk on row %i"
+				" -- please report a bug", row);
 	return 0;
+    }
 
     /* First, blank out the row. */
     blank_row(edit, row, 0, COLS);