fix: stabilise MenuPortal updateComputedPosition to prevent infinite re-render loop

[9KenM]

Problem

updateComputedPosition closes over computedPosition and lists its
sub-properties (offset, rect.left, rect.width) as useCallback
dependencies. This means every call to setComputedPosition produces a
new callback identity, which invalidates the
useLayoutEffect([updateComputedPosition]) and immediately re-fires the
callback.

Under subpixel-drifting conditions (zoomed-out viewport, scrollable
ancestor, or fractional DPI) the measured offset oscillates by <1px
between frames. Each oscillation triggers setComputedPosition →
identity change → effect re-run → repeat, until React throws:

Maximum update depth exceeded.

Fixes #6003.

Solution

Track computedPosition in a ref so the comparison inside
updateComputedPosition always reads the latest value without the
callback depending on it. This gives updateComputedPosition a stable
identity across renders and breaks the loop.

Reproduction

1. Render a portaled <Select menuPosition="fixed" /> inside a scrollable container
2. Set browser zoom to 90% or 110% (triggers subpixel rect drift)
3. Open the dropdown and scroll the container
4. Console shows: Maximum update depth exceeded

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