Improve FloatingMenu CSS so it stays with its parent scroll

[Keavon]

Before 8bc85bd774ff3349bbdc5eb652643bf69fee83e4, the CSS was set up to make floating menus employ a clever but fragile arrangement of position: flex, position: absolute, transform: translate(0), and other properties to make the elements stick to the bottom of their parent (without needing JS to position them) and also not get clipped by the overflow: hidden of their containing panel.

It was discovered that this arrangement did not hold up to the problem of scrolling the parent, such as the Properties panel containing widgets that spawn a floating menu (like ColorInput). 8bc85bd774ff3349bbdc5eb652643bf69fee83e4 used JS to fix this, but doing so has the drawback that it isn't recomputed automatically by the CSS engine when things change. Window resizes, scrolling the parent, and other changes can cause the position of the spawner to change but the JS won't update the floating menu.

When placing floating menus within other floating menus (specifically, a DropdownInput inside a DialogModal in #629), it became necessary to further complicate the JS logic by disabling the positioning behavior. But if a DialogModal ever has scrollable content in the future, disabling it won't fix the original scrolling problem.

If possible, investigate a primarily CSS-based solution that's robust to the challenges described above. Then revert the complex JS that was introduced in that commit. This might help: https://css-tricks.com/popping-hidden-overflow/