[popover] Expose an imperative updatePosition() API for Popover

[FotisVasilopoulos]

I'm using Popover with content that can change size after it has opened (for example, a "Show more" button that expands the content).

When the content expands, the popover continues using the position and available height that were calculated when it first opened. As a result, it can overflow the viewport instead of being repositioned or having its available height recomputed.

Floating UI already supports recomputing the position via update(), but I couldn't find a way to trigger this through Base UI's public API.

It would be useful if the imperative handle exposed something like:

handle.updatePosition();

This would allow consumers to request a recomputation after dynamic layout changes such as:

• Expanding/collapsing sections ("Show more")
• Async data loading
• Images loading
• Virtualized content changing size
• Content that changes height based on user interaction

Currently the only workaround I've found is to remount the Popover.Positioner (increasing its key), which resets the internal state of the popover content and isn't ideal.

[jjenzz]

Thanks for reporting this. I'm going to mark this as a bug for now because it seems that position updates caused by content/layout changes should be handled automatically rather than requiring consumers to call an imperative API.

We'll need to investigate why this isn't happening in these cases and whether we can make the existing positioning logic handle them.

[atomiks]

@FotisVasilopoulos can you link a reproduction or recording of what you're seeing?

Updating automatically is already covered by elementResize/layoutShift options — i.e. the popover's position updates when the anchor resizes, the popup resizes, or the anchor relocated on the screen due to layout shift. In all cases the popup should already update its position.

(There is #4537, but it's for integration with third party widgets like Maps/canvas for virtual elements, not general DOM scenarios)