Host TableView rows in an ItemsRepeater with a custom VirtualizingLayout

[w-ahmad]

Summary

TableView used to derive from ListView, which is where its row virtualization, container generation, row selection storage, and the Windows row visual all came from. That base class had become the limiting factor:

• Hit testing during drag selection scanned every item on every pointer-move frame and every 16 ms auto-scroll tick.
• Row positions were maintained with a TransformToVisual walk per row, per scroll frame.
• Cell selection materialised one entry per selected cell (SelectAll on a large grid allocated millions of structs).
• ScrollRowIntoView was a 10-try realization poll with a magic + 8 correction to work around ListView's async realization.
• Grouping was structurally impossible: the whole control assumed visual row == item index.

TableView now derives from Control and hosts its rows in an ItemsRepeater driven by a purpose-built VirtualizingLayout (TableViewRowsLayout). Realized row elements track the viewport height, not the item count — 200 items and 100,000 items in the same viewport realize the same number of rows. Row selection is a range-based model (TableViewSelectionModel), so selecting a million rows is one range and SelectedItems.Count is O(1). Hit testing, scroll-into-view, and PageUp/PageDown are index arithmetic against the layout's row-height store instead of scanning realized containers.

What's included

• TableView : Control (was : ListView), TableViewRow : ContentControl (was : ListViewItem), with TableViewRow now owning its own visual-state machine (CommonStates, DisabledStates, MultiSelectStates, pointer-cancel/capture-lost handling).
• The row template is unified across Windows and Uno (Windows previously used the native ListViewItemPresenter).
• New src/Layout/ (TableViewRowsLayout, TableViewRowHeights, TableViewRowElementFactory) and src/Selection/ (TableViewSelectionModel, TableViewSelectedItemsCollection, TableViewCellSlotCollection).
• Automation peers rewritten on FrameworkElementAutomationPeer with hand-written Selection/Scroll/ItemContainer providers. IGridProvider.GetItem now realizes off-screen rows on demand instead of returning null — fixes a real accessibility gap.
• Shift+click range selection, previously delegated to ListViewItem, is implemented directly.
• New sample page Virtualization: item-count switch (1k → 1M), select-all timing, jump-to-row, live realized-row counter that demonstrates the constant-row-count property.
• Docs rewritten for the new architecture (overview.md, performance.md, accessibility.md, selection.md, incremental-loading.md, migration-wct.md, README, docs/index.md) plus a new docs/docs/breaking-changes.md.

Fixed along the way

• An empty incrementally-loading source was not always asked for its first page.
• A row recycled mid-edit silently dropped the edit and left IsEditing stuck true with the corner menu permanently disabled; it now commits the edit before the row is released.
• A lost pointer capture (alt-tab, cancelled touch) left the table stuck in drag selection.
• IGridProvider.GetItem returning null for off-screen rows, making large tables effectively unreadable to assistive technology.

Breaking changes

This is a deliberate semver-major change — see docs/docs/breaking-changes.md for the full table. Highlights:

• CanDragItems/CanReorderItems row drag-reorder is not currently supported. It came entirely from ListViewBase with no code in this library, so it disappeared with the base class rather than being ported. ReorderRowsPage documents the gap; reimplementing it is tracked as follow-up work.
• ItemTemplate, ItemsPanel, ItemContainerStyle, Header/Footer, IsItemClickEnabled, and a handful of other inherited ListViewBase/ItemsControl members are gone. Everything this repo's own code, samples, and tests actually use was re-declared on TableView with the same name/type.

Test plan

• msbuild /restore /t:Build,Pack src/WinUI.TableView.csproj — green on all 6 target frameworks (net8/9/10, plain and -windows10.0.19041.0)
• WinUI.TableView.Tests — 361/361 pass
• WinUI sample app builds (x64)
• Uno sample builds for net10.0-desktop

Stacked PR

Multi-level grouping is a separate follow-up PR stacked on this one.

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