[BUGFIX] Correct overflow behavior for iframes in modal windows

Resolves: #81539
Releases: master, 8.7
Change-Id: I2633df64020e9f6a8e95048ed2d23f84d3013313
Reviewed-on: https://review.typo3.org/53184


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>