Add .fixed positioning modifier to x-anchor

[nicolagianelli]

What

Adds a new .fixed modifier to the x-anchor directive that instructs Floating UI to use its position: fixed strategy instead of the default position: absolute.

<div x-show="open" x-anchor.fixed="$refs.button">
    Dropdown content
</div>

Why

x-anchor currently hardcodes position: absolute when applying styles. This works for most cases, but it breaks down when:

• The reference element lives inside a container with overflow: hidden / overflow: clip / overflow: auto — the popover gets clipped along with the parent.
• You want the popover to stay pinned to the viewport regardless of scroll.

Floating UI natively supports a fixed strategy for exactly this — Alpine just didn't have a way to pass it through.

How

• getOptions() returns a new strategy: 'fixed' | 'absolute' field, derived from whether the .fixed modifier is present. Defaults to 'absolute', so existing x-anchor usage is byte-identical in behavior.
• computePosition() receives the strategy option.
• setStyles() takes the strategy as an argument (defaulting to 'absolute') and applies it as the CSS position value.
• The clone/morph branch also reads strategy from getOptions(modifiers) and forwards it to setStyles(). This is necessary for correctness: when .fixed is active, the (x, y) stored on _x_anchor are viewport-relative, so the pre-existing branch that called setStyles(el, x, y) would have applied viewport coordinates under position: absolute after a morph, placing the popover in the wrong spot. This PR implicitly fixes that.

Backward compatibility

Fully additive. strategy defaults to 'absolute' in every code path, and setStyles defaults its parameter to 'absolute' too. Any existing x-anchor expression without the .fixed modifier produces identical output before and after this change.

Documentation

• Tightens the Fixed positioning section's lede to describe when absolute falls short.
• Adds a prominent heads-up about the transformed-ancestor gotcha — if an ancestor has transform, filter, perspective, backdrop-filter, will-change, or contain, the CSS spec says it becomes a new containing block for position: fixed descendants, which is the Struggling with x-class binding #1 footgun when .fixed appears to do nothing.
• Adds a note under Manual styling explaining how to combine .no-style with .fixed correctly (the caller must apply position: fixed themselves, because $anchor.x/y are returned in the coordinate space of whichever strategy is active).

Tests

New Cypress tests in tests/cypress/integration/plugins/anchor.spec.js:

• .fixed applies position: fixed
• absence of .fixed keeps the default position: absolute (backward-compat regression guard)
• modifier order is interchangeable (.bottom.fixed === .fixed.bottom)
• .fixed still correctly anchors when the reference lives inside a position: relative; overflow: hidden clipping container
• .fixed strategy survives an Alpine.morph() pass — verifies the clone-branch setStyles call now correctly re-applies the strategy

All existing anchor tests continue to pass unchanged.

Scope

The .match-width modifier that was present in an earlier version of this PR has been removed. It is orthogonal to .fixed (works with either positioning strategy) and deserves its own PR where its inline-width override behavior and bundle cost can be reviewed on their own merits. A follow-up PR will reintroduce it with docs and tests.

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Original work by @nicolagianelli. Scope cleanup, tests, and docs expansion by @calebporzio.

[calebporzio]

thanks!