I was playing with the new "copy all text" feature, and stumbled upon one document where the copied text was truncated; see http://mirrors.ctan.org/info/lshort/english/lshort.pdf
The problem turns out to be that on [page 83](https://ftp.acc.umu.se/mirror/CTAN/info/lshort/english/lshort.pdf#page=83) the textLayer contains `\u0000` and apparently copying just stops when a null char is encountered.
To fix this we can simply use an existing helper function, and with this patch we're able to successfully copy all the text in that document.