[BUGFIX] Correct position of t3editor autocomplete

The position of the autocomplete list for the TypoScript was incorrect
when the editing iframe was scrolled.

Resolves: #77719
Releases: master, 7.6
Change-Id: Ib0d17e6bc648fc99b1e02dfb2a212c1b19993bcc
Reviewed-on: https://review.typo3.org/49706


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Martin Fox <kontakt@mlis.pl>
Tested-by: Martin Fox <kontakt@mlis.pl>
Reviewed-by: Łukasz Uznański
Tested-by: Łukasz Uznański
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>