fix(dom): detect visible content for slotted slot elements

[driskull]

Related Issue: #14270

Summary

This change fixes a nested slot edge case where visibility checks fail when a slot is assigned into another slot.

Problem

The helper that checks whether an element has visible content only inspected child nodes.
For slot elements, visible content can come from assigned nodes rather than direct children, so nested slot usage could be reported as empty.

Solution

Updated hasVisibleContent to handle slot elements by checking assigned nodes with flatten enabled.
If any assigned node is:

• non-whitespace text, or
• an element node

the helper now returns true.

Tests

Added coverage in dom.browser.spec.ts:

• should return true if slot element has assigned visible content

Verified with:
EXPERIMENTAL_TESTS=true npx vitest run src/utils/dom.browser.spec.ts

Impact

• Fixes visibility detection for nested slot scenarios
• Unblocks components relying on hasVisibleContent for slot-change state updates (including panel heading/description behavior)
• No API changes

[Copilot]

Pull request overview

Fixes hasVisibleContent for <slot> elements so visibility checks account for slotted/assigned nodes (including nested slots via flatten: true), preventing false “empty” results in nested slot scenarios.

Changes:

• Update hasVisibleContent to detect visible content from HTMLSlotElement.assignedNodes({ flatten: true }).
• Add a unit test covering the slot-assigned-content path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/components/src/utils/dom.ts	Adds slot-aware visible-content detection using flattened assigned nodes.
packages/components/src/utils/dom.browser.spec.ts	Adds test coverage for hasVisibleContent() when called with a slot element.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.