[Controls] Add drag-to-select marquee to GroupedTileListBox

[Xavier Fortin (xfortin-devolutions)]

Summary

Adds Windows Explorer-style rubber-band (marquee) selection to GroupedTileListBox, building on the multi-selection support from #598. Dragging over empty space draws a rectangle and selects every tile it covers, with auto-scroll when the pointer reaches the top/bottom edge.

Behavior

• Activation — active only when SelectionMode includes Multiple, and gated by a new opt-out EnableRectangleSelection property (default true). Single-select usages are unaffected.
• Modifiers — plain drag replaces the selection with the rectangle's contents; Ctrl/Cmd+drag adds (union). A plain click on empty space clears the selection (Explorer-style); Escape or lost pointer-capture reverts to the exact pre-drag selection (collection, primary, and range anchor).
• Auto-scroll — dragging past the top/bottom edge scrolls with a ramped speed and keeps extending the selection.

Implementation notes

• Overlay — a viewport-fixed Canvas template part (PART_SelectionRectangle) shares the ScrollViewer's origin, so it needs no coordinate reconciliation and never scrolls with content. Accent-tinted brushes are theme-aware via SystemAccentColor; the template lives only in the default theme (the per-theme files use BasedOn), so one edit covers all themes.
• Hit-testing is analytic, not container-based — a marquee is a 2D region test (a narrow vertical drag must select only one column, not a contiguous range), and items are virtualized, so off-screen tiles must still be testable. Rectangles are computed analytically from the layout.
• Single source of truth for tile geometry — a shared EnumerateItemLayout walk (headers, rows, inter-group spacing) is consumed by both the marquee hit-test and CalculateScrollOffsetForItem, so scroll positioning and selection can't drift apart.
• Selection is applied by diff-reconciling SelectedItems (reference equality, existing re-entrancy guard), and deliberately bypasses auto-scroll-on-selection so it doesn't fight the marquee's own scrolling.

Demo

SampleApp Scenario 5 ("Multiple Selection") now uses a dedicated multi-group dataset (8 groups, uneven counts ≈ 426 items) so groups span many rows and the list scrolls — exercising cross-group rectangles and edge auto-scroll. Its description was updated with the new gesture.

Follow-up fixes (review feedback + edge cases)

• Collection mutation mid-drag — OnCollectionChanged now aborts an in-progress marquee, so an in-place ItemsSource mutation can't leave a stale baseline/anchor that re-adds a removed item on the next move or on Escape.
• Secondary mouse buttons — a marquee now starts only on the initiating left-button press (and only when none is already in progress) and finishes only on that button's release, so a secondary button pressed/released mid-drag no longer resets or prematurely commits it.
• Exact-order cancel — cancelling a marquee restores the pre-drag selection in its exact original order.
• Right-click preserves multi-selection — right-clicking an item that is already selected no longer collapses the selection to that single item, so a context menu can act on every selected item; right-clicking an unselected item still selects it (replacing), and the press is left unhandled so the menu can open.

Testing

• ✅ Devolutions.AvaloniaControls and SampleApp build clean.
• ✅ App loads the demo page without runtime/template errors.
• ⚠️ Visual baselines: the GroupedTileListBoxDemo baselines need regenerating (per-OS, via CI) — the Scenario 5 description reflow and the new dataset change that page at rest. The overlay itself is IsVisible=false when idle, so it causes no other baseline changes.

Please give the marquee an interactive pass (drag across group headers; drag to an edge to confirm auto-scroll into later groups; Ctrl/Cmd+drag union; Escape revert; right-click within a multi-selection).

[Copilot]

Pull request overview

Adds marquee multi-selection with auto-scroll to GroupedTileListBox.

Changes:

• Implements rectangle selection, cancellation, and analytical hit-testing.
• Adds a themed selection overlay.
• Expands SampleApp’s multi-selection scenario.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
GroupedTileListBox.axaml.cs	Implements marquee behavior and geometry.
GroupedTileListBox.axaml	Adds the selection overlay.
GroupedTileListBoxViewModel.cs	Generates a large grouped dataset.
GroupedTileListBoxDemo.axaml	Updates the interactive demo.

Comments suppressed due to low confidence (3)

src/Devolutions.AvaloniaControls/Controls/GroupedTileListBox/GroupedTileListBox.axaml.cs:1737

• This accepts any press that occurs while the left button is already held. For example, a right-button press during a left-button marquee resets the pending gesture and its anchor. Check that this event itself is the left-button press.

        PointerPoint point = e.GetCurrentPoint(this.scrollViewer);
        if (!point.Properties.IsLeftButtonPressed)
        {
            return;

src/Devolutions.AvaloniaControls/Controls/GroupedTileListBox/GroupedTileListBox.axaml.cs:1788

• Any button release currently commits the active marquee and releases capture. Releasing a secondary mouse button while the left button remains down therefore ends the drag prematurely; only the release paired with the initiating left press should finish it.

    protected override void OnPointerReleased(PointerReleasedEventArgs e)
    {
        base.OnPointerReleased(e);

src/Devolutions.AvaloniaControls/Controls/GroupedTileListBox/GroupedTileListBox.axaml.cs:1995

• This only removes obsolete entries and appends newcomers; it does not restore orderedTarget order. For example, cancelling from original [A, B, C] after a marquee leaves [A, C] produces [A, C, B], so Escape does not restore the exact pre-drag collection promised by this change.

        // Add newcomers, preserving target order.
        foreach (object item in orderedTarget)
        {
            if (!ContainsByReference(selectedItems, item))
            {
                selectedItems.Add(item);
            }

---

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

src/Devolutions.AvaloniaControls/Controls/GroupedTileListBox/GroupedTileListBox.axaml.cs:2198

• This grouped path rebuilds GroupBy results and a List for every group on every pointer move and 16 ms auto-scroll tick, and HitTestItems continues through every item even when the marquee covers only a few visible rows. That makes drag cost proportional to the entire source on the UI thread, defeating virtualization for large lists. Cache/invalidate row geometry or index rows by Y so each update only examines groups/rows intersecting the marquee.

        foreach (IGrouping<string, object> group in this.GetOrderedGroups(this.ItemsSource.Cast<object>().GroupBy(groupSelector)))
        {
            List<object> groupItems = group.ToList();

[randy-but-a-ro]

🤖 Pull request was approved automatically: the AI review is complete and all its review threads are resolved. 🎉

Integration Details

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