Optimize getComputedStyle calls in ActionList layout method

[Copilot]

Cache horizontal spacing calculations to reduce getComputedStyle() calls from O(n) to O(1) in the ActionList layout method.

Changes

• Cache computed styles: Calculate padding/margin once for the first element instead of per-item
• Reuse cached value: All items share the same CSS classes, making this safe

Implementation

// Cache computed style values since they're consistent across all list items
let cachedHorizontalSpacing: number | undefined;

const itemWidths: number[] = this._allMenuItems.map((_, index): number => {
    const element = this.domNode.ownerDocument.getElementById(this._list.getElementID(index));
    if (element) {
        // ... measure content width ...
        
        // Only compute style values once for the first element, then reuse for all others
        if (cachedHorizontalSpacing === undefined) {
            const computedStyle = dom.getWindow(element).getComputedStyle(element);
            const paddingLeft = parseFloat(computedStyle.paddingLeft) || 0;
            const paddingRight = parseFloat(computedStyle.paddingRight) || 0;
            const marginLeft = parseFloat(computedStyle.marginLeft) || 0;
            const marginRight = parseFloat(computedStyle.marginRight) || 0;
            cachedHorizontalSpacing = paddingLeft + paddingRight + marginLeft + marginRight;
        }
        
        return width + cachedHorizontalSpacing;
    }
    return 0;
});

Addresses performance feedback on parent PR #286343.

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