Enable multiDrag only when a selection class is configured

[Kebechet]

Problem

let multiDrag = (typeof cssForSelection !== 'undefined');

_cssForSelection is an uninitialised string field and is only assigned when settings.MultiSelection is set. .NET marshals an unset string as JSON null, never undefined, and typeof null === 'object' — so this expression is always true. Every list is constructed with multiDrag: true and selectedClass: null, whether or not multi-selection was requested.

The Array.from(event.newIndicies) calls in onUpdate/onRemove currently only survive because of this: oldIndicies/newIndicies are added exclusively by the MultiDrag plugin's eventProperties(), so with multiDrag correctly off they are undefined and Array.from would throw.

Implementation

• multiDrag is now a truthiness check on cssForSelection, so null and '' both correctly disable it.
• selectedClass passes undefined instead of null when unset, so SortableJS applies its own default rather than a null class.
• newIndicies is guarded in both onUpdate and onRemove, falling back to [] — required now that multiDrag can actually be off.

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• BlazorSortableList.Lib/SortableList.razor.js

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[Copilot]

Pull request overview

This PR fixes multi-selection configuration for the SortableJS integration by ensuring MultiDrag is only enabled when a selection CSS class is actually configured, and by hardening event handling for cases where MultiDrag-specific event properties are absent.

Changes:

• Compute multiDrag via a truthiness check on cssForSelection (so null/'' correctly disable MultiDrag).
• Pass undefined (not null) for selectedClass when no selection class is configured.
• Guard event.newIndicies usage in onUpdate/onRemove to avoid Array.from(undefined) when MultiDrag is off.

Comments suppressed due to low confidence (1)

BlazorSortableList.Lib/SortableList.razor.js:129

• Same issue as onUpdate: if MultiDrag is off, oldDraggableIndex/newDraggableIndex may be missing and JS interop will attempt to pass undefined into OnRemoveJS(int, int, ...). Falling back to oldIndex/newIndex makes the handler robust across Sortable configurations/versions.

            let oldIndex = event.oldDraggableIndex;
            let newIndex = event.newDraggableIndex;

            // in multi selection mode we have newIndicies only
            // Only the MultiDrag plugin adds these, so they are undefined when multiDrag is off.

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[Kebechet]

I do not think this one holds. oldDraggableIndex/newDraggableIndex are core SortableJS state, not MultiDrag additions - they are module-level variables in src/Sortable.js, assigned in _prepareDragStart, _onDragOver and _onDrop, and included in every dispatched event regardless of which plugins are mounted.

The genuinely MultiDrag-only properties are oldIndicies/newIndicies, which come from the plugin's eventProperties() - and those are exactly what this PR guards. Leaving the draggable indices as they are.