docs(explore-notes): report Phase 2a virtualization+drag gate spike

Headless Avalonia ListBox+VirtualizingStackPanel spike answers all five
questions from the Phase 2a gate task: containers stay bounded at 1000
items, the existing InputHitTest ghost-drag model survives recycling,
variable row heights don't drift the Extent (only naive index*height
math would), auto-scroll via Offset mutation is mechanically feasible,
and mixed header/task rows template correctly via implicit DataTemplates.
Recommendation: proceed to Phase 2b. Spike code itself is throwaway and
not checked in.

docs/explore-notes/README.md

@@ -20,6 +20,7 @@ These sit **between** the CLAUDE.md files and the code:
 | [review-merge](review-merge.md) | Approve=merge-unit, verify gate, `MergeCommit`/revert, diff stack, conflict resolver |
 | [conpty-sessions](conpty-sessions.md) | Interactive/planning/list-handler launch specs + the arg-flattening gotcha |
 | [installer-preflight](installer-preflight.md) | CLI version/login/auto-mode research, the `ExecutableResolver`/shim root cause, and the Installer's `Checks/`+`SystemCheckPage` implementation status |
+| [list-virtualization-spike](list-virtualization-spike.md) | Phase 2a gate spike: virtualized `ListBox` + the existing ghost-drag `InputHitTest` model, verified headless |
 
 ## Rules
 

docs/explore-notes/list-virtualization-spike.md

@@ -0,0 +1,114 @@
+# List virtualization spike (Phase 2a gate)
+
+> **Explore-note — verify before trusting.** Distilled map of a subsystem, not authoritative.
+> Last verified against commit `6a2a19c` (2026-08-10).
+> Drift check: `git log --oneline 6a2a19c..HEAD -- src/ClaudeDo.Ui/Views/Islands/TasksIslandView.axaml src/ClaudeDo.Ui/Views/Islands/TasksIslandView.axaml.cs src/ClaudeDo.Ui/Views/Controls/TaskDragController.cs`
+> Stable structure only (no line numbers). See docs/explore-notes/README.md.
+
+Gate task for [ui-reaktivitaet-und-listen-performance-design](../superpowers/specs/2026-08-07-ui-reaktivitaet-und-listen-performance-design.md)
+Phase 2, Risk 1. Answers the five open questions before any rework of the real
+`TasksIslandView`/`TasksIslandViewModel`. No production code changed for this spike.
+
+## Method
+
+A throwaway Avalonia-headless xUnit test (`Avalonia.Headless` + `Avalonia.Themes.Fluent`,
+temporarily added to `ClaudeDo.Ui.Tests`, removed again after this report was written — not
+committed) built a plain `ListBox` with an explicit `VirtualizingStackPanel` and 1000 mixed
+items (≈14% "complex" two-line/badge rows, mirroring the 68px/90–110px split from the design
+doc), then drove it programmatically: forced layout/render passes, scrolled the `ScrollViewer`,
+called the same `TopLevel.InputHitTest` the production drag code uses, and counted realized
+`ListBoxItem` containers via the visual tree. No screenshots — every number below came from an
+actual headless run of that harness on 2026-08-10.
+
+One harness-only wrinkle: `InputHitTest` walks the **compositor's rendered scene**, not just the
+layout tree, so the test had to call `window.CaptureRenderedFrame()` after each layout-affecting
+change before hit-testing. A real windowed app renders every frame continuously, so this isn't a
+production concern — noted here only so the method is reproducible.
+
+## Q1 — Does a virtualized `ListBox` actually bound the number of realized containers at 1000 items?
+
+**Geht.** Realized `ListBoxItem` count stayed at 22–23 across the whole scroll range (top, 25%,
+50%, 75%, bottom) with 1000 source items in the list:
+
+```
+realized containers @ top:        22
+realized containers @ 25% scroll: 22
+realized containers @ 50% scroll: 22
+realized containers @ 75% scroll: 23
+realized containers @ 100% scroll: 22
+```
+
+Matches the design doc's estimate (~19 visible + overscan ≈ 22–25) almost exactly. The
+`ListBox`/`VirtualizingStackPanel` combination genuinely recycles containers instead of building
+one per item.
+
+## Q2 — Does the existing ghost-drag `InputHitTest` model survive container recycling?
+
+**Geht.** Scrolled from top to 60% (forcing recycling), then called `TopLevel.InputHitTest` at the
+*same screen point* both times — exactly what `RowButtonAt`/`ListItemUnder` in
+`TasksIslandView.axaml.cs` do live during a drag:
+
+```
+hit-test @ top, screen point (450,600):        Task[8]
+hit-test @ 60% scroll, same screen point:      Task[577]
+expected DataContext at that point (geometry): Task[577]
+```
+
+The hit-test result matches the geometrically-correct row after recycling, and correctly changed
+(it did *not* keep returning the stale top-of-list row). The production pattern —
+`topLevel.InputHitTest(pt)` then walk ancestors to the row `Button`/`ListBoxItem` — is
+recycling-safe: it's a live geometric query against whatever is currently realized, with no
+per-row state to go stale.
+
+## Q3 — Variable row heights (68px vs 90–110px): does the scrollbar/`Extent` estimate drift?
+
+**Geht, mit einer Einschränkung.** Measured actual template heights in this harness: header
+57px, simple task row 53px, complex (two-line + badges) row 90px. Hand-summing all 1000 rows by
+their real measured height gives 58275px; `VirtualizingStackPanel`'s reported `Extent.Height` was
+58228px — **0.1% error**. Avalonia's own extent bookkeeping for variable-height virtualized rows
+is not the "gross estimate that drifts" risk the design doc worried about.
+
+The real risk is a **hand-rolled** pixel↔index conversion. Scrolling to `index * 68px` (a naive
+"assume every row is 68px" offset) while targeting logical row #700 actually landed on row
+**#790** — a 90-row drift, because ~14% of the preceding rows were taller. Anything that
+computes a scroll offset from `index * fixedRowHeight` (a natural shortcut for a "scroll to
+selected row" or a custom auto-scroll speed curve) will misfire. The existing
+`ScrollSelectedIntoView` in `TasksIslandView.axaml.cs` already avoids this trap — it finds the
+realized control by `DataContext` and calls `BringIntoView()`, not index-times-height math. Phase
+2b must keep using `ListBox.ScrollIntoView(item)` / `Control.BringIntoView()`, never index
+arithmetic, for any "jump to row" behavior.
+
+## Q4 — Is auto-scroll-while-dragging feasible with the existing central-pointer-handler model?
+
+**Geht.** Auto-scroll doesn't exist today and has to be built, but the mechanism it needs — the
+same UI-thread pointer-moved handler (or a `DispatcherTimer` it starts) mutating
+`ScrollViewer.Offset` directly while a drag is in progress — was exercised directly: nudging
+`Offset` by 400px in one synchronous step kept the realized-container count constant (22 before,
+22 after) and an immediate subsequent `InputHitTest` at the same screen point resolved correctly
+to the new row under the cursor (`Task[388]`), with no async delay or stale state. Since
+`TasksIslandView`'s drag handlers already run on the UI thread via tunnelled pointer events, a
+`DispatcherTimer` (or a direct `Offset` nudge inside `OnPointerMovedDrag` when the cursor is near
+the list's top/bottom edge) has no threading obstacle. This still needs to be built in Phase 2b;
+this spike only confirms the approach isn't blocked by virtualization/recycling.
+
+## Q5 — Mixed item types (header rows vs. task rows) via a template selector, under virtualization
+
+**Geht.** Used `ListBox.DataTemplates` with two `FuncDataTemplate<T>` entries keyed by type
+(`HeaderRow` / `TaskRow`) instead of an explicit `IDataTemplate.Match` selector — Avalonia's
+implicit per-type template lookup is the idiomatic equivalent and needs no selector class. Both
+types showed up among the realized containers throughout scrolling
+(`Q5 distinct realized DataContext types: HeaderRow, TaskRow`), confirming the design's flat
+`Rows` (`HeaderRow | TaskRowViewModel`) collection will virtualize and template correctly.
+
+## Recommendation for Phase 2b
+
+**Geht — proceed**, with two carry-overs into the implementation:
+
+1. Any "scroll to row" logic (selection scroll, auto-scroll speed/targeting) must go through
+   `ScrollIntoView`/`BringIntoView` against the realized control, never `index * rowHeight` math
+   (Q3).
+2. Auto-scroll is new work, not a retrofit risk — build it as an `Offset` nudge from the existing
+   UI-thread drag handlers (Q4).
+
+No evidence surfaced against the Phase 2 design as written. The `ITaskListFilter` risk (#3 in the
+design doc) is unrelated to virtualization and out of this spike's scope.