Thanks for mentioning that.
This has to do with trying to read entries that do not exist. (Looking for the button above a button on the most upper row of buttons does not make sence right??)

I did not encounter this problem at the time I submitted this, as this was allowed by PHP and would result in a 'null' value (hence the check if the result is not null).

Replacing the checks with a more PHP-friendly way should do the trick, so line 8 would be:
if($j > 0) $vals[] = $notepad[$i][$j-1]
This is less pretty and satisfying to look at, but oh well...

with PHP8 you get Undefined array key -1 (line 8)

Compact, but hard to read.

When you get your comment wrong because youre searching for "snip" in the windows menu but somehow it also types it in the HTML editor (see row 13)......
NOW I CANNOT EDIT THIS ANYMORE