feat: allow drag selection to start from rows and empty space

[w-ahmad]

Summary

Improves the drag-selection experience in TableView that was added in PR #344 by allowing selection gestures to begin from row containers and empty space — not just individual cells. This makes the control feel much more natural and consistent with other data-grid implementations.

Recording.2026-07-30.034511.mp4

Changes

New Types

Type	Description
TableViewCellSlotRange	A coordinate-based, rectangular range of cell slots (first/last row + column). Includes Equals, GetHashCode, equality operators, and factory helpers (FromCoordinates, FromSlots).
TableViewCellSlotRangeExtensions	Extension methods for TableViewCellSlotRange: IsInRange, IsRowInRange, IsColumnInRange, IsValid, GetSlots, Contains, IntersectsWith, Subtract, Merge.
IndexRangeHelper	Utility helper for index-range arithmetic used internally during selection.
TableViewTrace	[Conditional("DEBUG")] trace helper that writes tagged messages to Debug.WriteLine.

Core Changes

• TableView.cs – Major rework of drag-selection hit-testing so that pointer-down events on TableViewRow containers and blank list areas correctly initiate a selection rectangle, with full row/column range tracking.
• TableViewCell.cs – Selection-related logic extracted out of the cell and centralised in TableView, reducing the cell to its display responsibilities.
• TableViewRow.cs – Removed redundant pointer-event handling that is now owned by TableView.
• TableViewCellSlot.cs – Minor adjustments to align with the new range types.
• ItemIndexRangeExtensions.cs – Extended to support the new selection model.

Tests

• DragSelectionRectangleTests.cs – New tests covering drag-selection rectangle geometry.
• TableViewSelectionUnitTests.cs – Updated to cover selection scenarios that start outside individual cells.

Sample App

• OverviewPage and ExampleModelColumnsHelper updated to demonstrate the improved drag-selection behaviour.

Checklist

• New public types have full XML documentation
• Nullable reference types respected throughout
• Debug-only tracing via TableViewTrace (no production overhead)
• Unit tests added / updated
• Tested on WinUI 3
• Tested on Uno Platform

[Copilot]

Pull request overview

This PR enhances TableView’s drag-selection so selection gestures can start from row containers and empty list space (not only cells), while centralizing selection/drag logic into TableView and introducing coordinate-based range helpers to support rectangular cell selection.

Changes:

• Introduces TableViewCellSlotRange (+ range extensions) and index-range helpers to model rectangular cell selections and perform range arithmetic.
• Reworks pointer/drag-selection handling in TableView to support starting drags from rows and empty space, and refactors selection state updates.
• Updates and adds tests to cover range-based selection APIs (SelectCellRange / DeselectCellRange) and drag-selection behaviors.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/TableViewSelectionUnitTests.cs	Updates selection tests to use range-based APIs.
tests/DragSelectionRectangleTests.cs	Adds tests for SelectCellRange / DeselectCellRange event behavior and selection results.
src/TableViewRow.cs	Removes row-level pointer handling now owned by TableView.
src/TableViewCellSlotRange.cs	Adds new public range type representing rectangular cell-slot selections.
src/TableViewCellSlot.cs	Adds ToString() for improved diagnostics/logging.
src/TableViewCell.cs	Removes selection/drag logic from cell and adjusts pointer/edit handling.
src/TableView.Properties.cs	Migrates internal selection storage to HashSet<TableViewCellSlotRange>.
src/TableView.Events.cs	Adds debug tracing for cell selection changed notifications.
src/TableView.cs	Major drag-selection + hit-testing rework; adds range-based selection/deselection APIs and selection-state dispatching.
src/Helpers/TableViewTrace.cs	Adds DEBUG-only tagged tracing helper.
src/Helpers/IndexRangeHelper.cs	Adds helper to build contiguous ItemIndexRange sequences from indexes.
src/Extensions/TableViewCellSlotRangeExtensions.cs	Adds range operations (contains/intersects/subtract/merge, slot enumeration, validity).
src/Extensions/ItemIndexRangeExtensions.cs	Adds Contains/Subtract and refines IsValid signature for selection logic.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (4)

src/TableView.cs:325

• During row drag-selection, CurrentRowIndex (and SelectedIndex) are never updated when selection is driven by SelectRange/DeselectRange. Keyboard navigation uses CurrentRowIndex when LastSelectionUnit is Row, so after a drag-select the arrow keys can start navigating from a stale row index.

        if (LastSelectionUnit is not TableViewSelectionUnit.Cell && GetRowIndexAtCanvasPoint(_lastDragCanvasPoint.Value) is int row)
        {
            SelectionStartRowIndex ??= row;
            var minRow = Math.Min(SelectionStartRowIndex.Value, row);
            var maxRow = Math.Max(SelectionStartRowIndex.Value, row);

src/TableView.cs:1635

• OnCellSelectionChanged sets _cellStateDispatchPending = true and then ignores the return value of DispatcherQueue.TryEnqueue. If enqueue fails (e.g., dispatcher shutting down), _cellStateDispatchPending stays true and pending row visual updates will never run again.

        if (!_cellStateDispatchPending)
        {
            _cellStateDispatchPending = true;
            DispatcherQueue.TryEnqueue(ApplyPendingCellStates);
        }

src/Extensions/ItemIndexRangeExtensions.cs:62

• Subtract() returns an empty sequence when the ranges don’t overlap. For subtraction semantics, “no overlap” should normally return the original range unchanged; returning nothing can make callers accidentally drop the entire range.

        // No overlap.
        if (otherEnd < start || otherStart > end)
        {
            yield break;
        }

src/TableView.cs:235

• Local variable names in OnAnyPointerPressed contain typos (isShiftkey, orignalSoruce). This makes the code harder to read and propagates the typo into subsequent checks.

        var ctrlKey = KeyboardHelper.IsCtrlKeyDown();
        var isShiftkey = KeyboardHelper.IsShiftKeyDown();
        var orignalSoruce = e.OriginalSource as FrameworkElement;