TYPO3 Core

solution?!

delete this:

.x-menu-floating {
z-index: 50 !important;
}

in:
sysext/t3skin/Resources/Public/Css/backend.css

Update:

To reproduce this:

Change in the extension "rtehtmlarea" the configuration to "demo" and then create a new table, you see there the "background-option"

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/44755