Skip empty DocumentFragment append visual work #562

Parent: #243. CSS/performance epic: #235.

Proven hot path

Pinned unchanged Code OSS sanitizes selected Chat fragments and then unconditionally appends them. Sanitization can remove all nonportable links/images, leaving an empty DocumentFragment; WebScene still recascaded/marked dirty/published for that no-op. Batched panel and color-picker fragment append sites have the same shape.

Scope

After existing receiver/type/cycle handling, return the original DocumentFragment when its child list is empty, before recascade, dirty publication or lifecycle work. This matches the DOM insertion algorithm's zero-node early return. Preserve every real fragment detach/adopt/insert, focus/selection, mutation, custom-element, selector, identity and publication behavior.

Acceptance

• Focused source and regression authored.
• git diff --check passes.
• 4,096 connected and 4,096 detached empty-fragment appends preserve return/fragment/row/focus/selection/lifecycle/style/node-count state with zero recascade/layout/publications.
• 256-row nonempty control and detach/reattach produce exact callbacks and one bounded layout/publication; cleanup reclaims memory.
• Code OSS A/B/ABBA, browser/package, pixel, physical-performance and memory gates run later.

No build, test, benchmark or formatter was executed.