MudPopover: Keep spawner popovers cross-axis aligned near viewport edges

[Copilot]

MudSelect and similar spawner components could become visually detached from their trigger near viewport edges because overflow padding was clamping both axes indiscriminately. This caused horizontal drift (for vertical dropdowns) introduced with newer popover positioning behavior.

• Positioning behavior change (mudPopover.js)

  • Added effective placement classification after flip resolution.
  • Detects whether the popover is an anchor-aligned vertical dropdown (mud-popover-top-* / mud-popover-bottom-*) or a horizontal spawner (mud-popover-center-left/right).

• Cross-axis overflow clamp gating

  • Skipped left clamp when placement is horizontally anchor-aligned (top/bottom variants), preserving x-alignment with the trigger.
  • Skipped top clamp when placement is vertically anchor-aligned (center-left/right variants), preserving y-alignment with the trigger.
  • Kept existing overflow/flip behavior for the expansion axis and non-spawner placements.

• Net effect

  • Dropdown-style popovers remain connected to their spawn element even near edges, while existing edge handling and flip logic remain in place.

const alignsHorizontallyToAnchor = effectiveClassListArray.some((className) =>
    className.startsWith('mud-popover-top-') || className.startsWith('mud-popover-bottom-'));

if (!alignsHorizontallyToAnchor &&
    left + offsetX < window.mudpopoverHelper.overflowPadding) {
    left = window.mudpopoverHelper.overflowPadding;
    offsetX = 0;
}