Deliver each drop to the zone under the cursor

app/MindWork AI Studio/Components/DropZoneScope.razor.cs

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+using Microsoft.AspNetCore.Components;
+
+namespace AIStudio.Components;
+
+/// <summary>
+/// Marks an area whose drops end up at its default target when they hit no specific zone.
+/// </summary>
+/// <remarks>
+/// <para>
+/// This is how the habitual behaviour survives the move to hit testing: a file dropped anywhere in
+/// the chat, in an assistant, or in a dialog still arrives where it used to, while a file dropped
+/// on a specific zone now arrives exactly there. No code decides between the two -- the browser
+/// does, because the specific zone lies deeper in the DOM than the area around it, and the hit test
+/// resolves from the inside out.
+/// </para>
+/// <para>
+/// The component replaces an existing element rather than adding one: it takes the class and the
+/// style of the element it stands in for. Where the areas already had a wrapper, which is the case
+/// for every page and every assistant, nothing about the layout changes.
+/// </para>
+/// </remarks>
+public partial class DropZoneScope : ComponentBase
+{
+    /// <summary>
+    /// The content of the area.
+    /// </summary>
+    [Parameter]
+    public RenderFragment? ChildContent { get; set; }
+
+    /// <summary>
+    /// The CSS classes of the element this scope renders.
+    /// </summary>
+    [Parameter]
+    public string Class { get; set; } = string.Empty;
+
+    /// <summary>
+    /// The inline style of the element this scope renders.
+    /// </summary>
+    [Parameter]
+    public string Style { get; set; } = string.Empty;
+
+    /// <summary>
+    /// The state to cascade, for hosts which want to be the default target of their own area.
+    /// </summary>
+    /// <remarks>
+    /// A host cannot read what it cascades itself, so a page which is its own drop target has to own
+    /// the state instead. The plugins page is such a case: it accepts an archive anywhere on it and
+    /// has no inner zone to hand the role to. Everybody else leaves this alone and lets the scope
+    /// keep its own state.
+    /// </remarks>
+    [Parameter]
+    public DropZoneScopeState? State { get; set; }
+
+    private readonly DropZoneScopeState ownState = new($"drop-zone-scope-{Guid.NewGuid():N}");
+
+    private DropZoneScopeState EffectiveState => this.State ?? this.ownState;
+}