Anchored floating content should support fallback sides before clamping

[alexstyl]

When anchored floating content is placed on a preferred side that does not have enough room, the current behavior appears to keep that side and clamp the final position into the viewport.

This can cause the floating content to overlap the anchor instead of appearing on the opposite side.

Example:

• preferred side is Start
• the anchor is already close to the left edge
• instead of placing the content on End, the content is clamped into the window and ends up covering the anchor

This is especially noticeable for tooltips, menus, and other anchored surfaces where overlap looks incorrect.

A better default would be:

1. compute the preferred placement
2. if it does not fit on the requested side, try the opposite side
3. if multiple candidates are possible, choose the one with the least overflow
4. only clamp as a last resort after selecting the best side

It would also be useful if this behavior were configurable, for example:

• keep current clamp-only behavior
• allow automatic fallback to opposite side
• optionally provide a list of fallback sides

That would make anchored positioning behave more like expected floating UI placement near viewport edges.

[alexstyl]

I tried implementing this but there is a catch.

Transitions might look weird and probably need updating from the caller side once the side gets updated.

ie, the tooltip is meant to appear above the anchor, but it gets swapped to render below the anchor due to clamping. The transition animating bottom top looks off, once the tooltip is to be displayed below the anchor.

Not sure how to address this yet. Might need to change the API to support this.