Revert "Popup: Hide popup when offscreen. Fixes #5892."

This reverts commit 556a4e3304f30b25c312253474d98ee069a52cbf.
Revert "Custom select: Must un-hide popup container before calculating height."

This reverts commit 541f963abfe2e228394ae4577a718b8c28f8a348.
Revert "Tests: Unit: Popup: Make sure closed popup is not :visible and that open popup is :visible. Tests #5892."

This reverts commit b69b430f5f0a740fdfb8223f089ab6feea17e06d.
Popup: Set visibility: hidden instead of display: none to remove the need for un-hiding before height calculation.

This uses the solution mentioned in #5912 instead of the original, display: none -based solution for #5892.