Deliver each drop to the zone under the cursor

app/MindWork AI Studio/Tools/DropZoneScopeState.cs

@@ -0,0 +1,57 @@
+namespace AIStudio.Tools;
+
+/// <summary>
+/// The shared state of one drop zone scope, meaning one page, one assistant, or one dialog.
+/// </summary>
+/// <remarks>
+/// A scope is the area whose drops end up at its default target whenever the cursor is not over a
+/// more specific zone. That is what users are used to: a file dropped anywhere in the chat hangs
+/// itself on the composer. This object connects the two sides -- the scope cascades it inwards, and
+/// the zone which wants to be the default target claims it here.
+/// </remarks>
+/// <param name="scopeId">The ID of the element the scope renders.</param>
+public sealed class DropZoneScopeState(string scopeId)
+{
+    /// <summary>
+    /// The ID of the element the scope renders. The hit test reports it for every point inside the
+    /// area which no more specific zone covers.
+    /// </summary>
+    public string ScopeId { get; } = scopeId;
+
+    private object? defaultZone;
+
+    /// <summary>
+    /// Makes the given zone the default target of this scope, unless another zone was there first.
+    /// </summary>
+    /// <remarks>
+    /// Zones initialize in render order, so the first one in the markup wins. Two zones asking for
+    /// the same area is a mistake in the markup rather than a state worth resolving, and taking the
+    /// first one is at least a rule which can be stated and logged. Asking twice is no mistake,
+    /// though: a zone whose parameters are set anew has to keep the role it already holds.
+    /// </remarks>
+    /// <param name="zone">The zone that wants to be the default target.</param>
+    /// <returns>True if the zone is the default target of this scope from now on.</returns>
+    public bool TryBecomeDefaultZone(object zone)
+    {
+        if (this.defaultZone is not null && !ReferenceEquals(this.defaultZone, zone))
+            return false;
+
+        this.defaultZone = zone;
+        return true;
+    }
+
+    /// <summary>
+    /// Gives the role of the default target up again so that another zone can take it.
+    /// </summary>
+    /// <remarks>
+    /// Every zone that took the role has to do this when it is disposed. Without it, an area would
+    /// lose its default target for good as soon as the zone holding it is created anew -- which is
+    /// what happens on every navigation and every time a dialog is opened again.
+    /// </remarks>
+    /// <param name="zone">The zone that gives the role up.</param>
+    public void ReleaseDefaultZone(object zone)
+    {
+        if (ReferenceEquals(this.defaultZone, zone))
+            this.defaultZone = null;
+    }
+}