#5306 (flicker): while a parent WebUI session is the active/streaming session,
a linked delegate subagent child that transiently reports message_count===0
between /api/sessions polls was dropped by _sidebarRowHasVisibleMessages BEFORE
_attachChildSessionsToSidebarRows could stack it under its parent. It never
entered sessionsRaw, so the row vanished, then reappeared on the next refresh
once its list metadata caught up — the flicker. Extend the visibility predicate
with an active-parent exception (mirroring the existing active-session
exception): a child_session whose parent_session_id is the active sidebar
session stays visible even at message_count 0. Scoped to the active parent so
truly-empty unrelated sessions are still hidden.
#5305 (orphan): a delegated subagent child whose WebUI parent is filtered out of
the current render (project/profile/source scope) was promoted to a contextless
top-level "Subagent Session" orphan. Suppress cross-surface child_session rows
whose parent row is absent from the render instead of orphaning them, mirroring
the existing archived-hidden-parent suppression (#4293). The genuinely-external
parent case (messaging/CLI) still orphans via the parentIsExternal branch.
Tests: tests/test_5306_subagent_sidebar_flicker.py (7 tests) locks both
invariants and the no-regression guards, executing the real sessions.js helper
regions under node like the existing lineage tests.
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