fix(app): anchor revealed tools under the sticky headers

[Hona]

Picking a shell in the N running menu opens its Used group and the shell, then scrolls to it. When the group was far above or taller than the screen, the shell landed off-screen.

Before	After

Why it missed

revealPart aimed at the wrong thing at the wrong time:

• It centered the whole group row, not the shell. A 43-tool group is taller than the viewport, so its center is far from the shell.
• scrollToIndex aims once. The group expands, the shell loads output, and rows above get measured after that, so the target moved after the scroll ended.

Fix: anchor the shell's top edge

The shell's top edge is the one stable point: its own growth (expanding, streaming output) extends downward. The reveal sets a scroll anchor on it, the same idea as CSS scroll anchoring, but for a tool inside a virtualized row.

flowchart TD
  R[reveal] --> B["batch: open group + shell, anchor = row key + part ID"]
  B --> M["rangeExtractor keeps the anchored row rendered → shell is in the DOM now"]
  M --> Z["resizeItem(row, offsetHeight) → timeline has its opened height now"]
  Z --> U[onUnpin] --> P[pinAnchor]
  RO["any row size commit (ResizeObserver → resizeItem batch)"] --> P
  P --> S["scroll so shell top = its scroll-margin-top"]
  X["wheel · touch · pointer · scroll key · scrollbar · other navigation · pinned"] --> REL[release anchor]

Loading

• Synchronous first jump. The anchored row is forced into the virtualizer range, like the active message row already is. Changing rangeExtractor re-runs solid-virtual's setOptions computed, so the shell renders in the same tick, wherever it is.

• Real geometry before aiming. Until its ResizeObserver delivers, the opened row keeps its collapsed size. The timeline may not scroll at all yet (unpin() ignores a timeline that cannot scroll) or ends above the shell. Committing the measured size first fixes both:

  const opened = virtualContent?.querySelector<HTMLElement>(`[data-index="${found.index}"]`)
  
  if (opened) resizeItem(found.index, opened.offsetHeight)
  input.onUnpin()
  pinAnchor()

• Event-driven, not timed. Every committed row size change re-pins the anchor in the resizeItem microtask, before paint. It covers rows above being measured and siblings above the shell inside the group growing.

• Reaching the end does not resume following. Aiming at a shell near the end lands on the end of the timeline. handleListScroll would treat that as "follow new content again", and the next streamed part would scroll the shell away:

  if (maxScroll <= 1 || (atEnd && arrived && !rendering.anchor)) input.onPin()

• Released by intent. User scroll input, other navigation (prepareNavigation), or any explicit pin (for example, a session switch) releases it. The anchor never fights the user.

• Where "top" is. scroll-margin-top: var(--sticky-accordion-top) on tool-part-wrapper. The page header and the stuck Used N header already set that variable, so the shell lands just under them, not behind them.

Route reveals (picking the parent's shell from inside a subagent) use the same path.

[Copilot]

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