Auto set active dockable for root dock

samples/DockControlPanelsSample/MainView.axaml

@@ -23,7 +23,7 @@
             <Factory />
           </DockControl.Factory>
 
-          <RootDock Id="ProportionalRoot" IsCollapsable="False" ActiveDockable="ProportionalMain">
+          <RootDock Id="ProportionalRoot" IsCollapsable="False">
             <ProportionalDock x:Name="ProportionalMain" Id="ProportionalMain" Orientation="Horizontal">
 
               <!-- Left Tool -->
@@ -68,7 +68,7 @@
             <Factory />
           </DockControl.Factory>
 
-          <RootDock Id="StackRoot" IsCollapsable="False" ActiveDockable="StackMain">
+          <RootDock Id="StackRoot" IsCollapsable="False">
             <StackDock x:Name="StackMain" Id="StackMain" Orientation="Horizontal" Spacing="5">
 
               <ToolDock Id="StackTool1" Alignment="Unset">
@@ -107,7 +107,7 @@
             <Factory />
           </DockControl.Factory>
 
-          <RootDock Id="GridRoot" IsCollapsable="False" ActiveDockable="GridMain">
+          <RootDock Id="GridRoot" IsCollapsable="False">
             <GridDock x:Name="GridMain" Id="GridMain" ColumnDefinitions="*,Auto,2*,Auto,*" RowDefinitions="Auto,*">
 
               <!-- Top row toolbar -->
@@ -159,7 +159,7 @@
             <Factory />
           </DockControl.Factory>
 
-          <RootDock Id="WrapRoot" IsCollapsable="False" ActiveDockable="WrapMain">
+          <RootDock Id="WrapRoot" IsCollapsable="False">
             <WrapDock x:Name="WrapMain" Id="WrapMain" Orientation="Horizontal">
 
               <ToolDock Id="WrapTool1" Alignment="Unset">
@@ -210,7 +210,7 @@
             <Factory />
           </DockControl.Factory>
 
-          <RootDock Id="UniformRoot" IsCollapsable="False" ActiveDockable="UniformMain">
+          <RootDock Id="UniformRoot" IsCollapsable="False">
             <UniformGridDock x:Name="UniformMain" Id="UniformMain" Rows="2" Columns="3">
 
               <ToolDock Id="UniformTool1" Alignment="Unset">
@@ -261,7 +261,7 @@
             <Factory />
           </DockControl.Factory>
 
-          <RootDock Id="DockRoot" IsCollapsable="False" ActiveDockable="DockMain">
+          <RootDock Id="DockRoot" IsCollapsable="False">
             <DockDock x:Name="DockMain" Id="DockMain" LastChildFill="True">
 
               <!-- Top toolbar -->

src/Dock.Model/Controls/IGridDockSplitter.cs

@@ -8,7 +8,7 @@ namespace Dock.Model.Controls;
 /// Grid splitter dock contract.
 /// </summary>
 [RequiresDataTemplate]
-public interface IGridDockSplitter : IDockable
+public interface IGridDockSplitter : ISplitter
 {
     /// <summary>
     /// Gets or sets resize direction.

src/Dock.Model/Controls/IProportionalDockSplitter.cs

@@ -8,7 +8,7 @@ namespace Dock.Model.Controls;
 /// Proportional dock splitter contract.
 /// </summary>
 [RequiresDataTemplate]
-public interface IProportionalDockSplitter : IDockable
+public interface IProportionalDockSplitter : ISplitter
 {
     /// <summary>
     /// Gets or sets whether the splitter allows resizing.

src/Dock.Model/Controls/ISplitter.cs

@@ -0,0 +1,12 @@
+// Copyright (c) Wiesław Šoltés. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for details.
+using Dock.Model.Core;
+
+namespace Dock.Model.Controls;
+
+/// <summary>
+/// Splitter contract.
+/// </summary>
+public interface ISplitter : IDockable
+{
+}

src/Dock.Model/FactoryBase.Dockable.cs

@@ -58,7 +58,7 @@ public virtual void RemoveDockable(IDockable dockable, bool collapse)
         if (dock.VisibleDockables.Count > 0)
         {
             var nextActiveDockable = dock.VisibleDockables[indexActiveDockable];
-            dock.ActiveDockable = nextActiveDockable is not IProportionalDockSplitter ? nextActiveDockable : null;
+            dock.ActiveDockable = nextActiveDockable is not ISplitter ? nextActiveDockable : null;
         }
         else
         {
@@ -98,21 +98,21 @@ protected void CleanupOrphanedSplitters(IDock dock)
             var dockables = dock.VisibleDockables.ToList();
 
             // If we only have splitters, remove all of them
-            if (dockables.Count > 0 && dockables.All(d => d is IProportionalDockSplitter))
+            if (dockables.Count > 0 && dockables.All(d => d is ISplitter))
             {
                 toRemove.AddRange(dockables);
             }
             else
             {
                 // Remove splitters that are at the beginning
-                while (dockables.Count > 0 && dockables[0] is IProportionalDockSplitter)
+                while (dockables.Count > 0 && dockables[0] is ISplitter)
                 {
                     toRemove.Add(dockables[0]);
                     dockables.RemoveAt(0);
                 }
 
                 // Remove splitters that are at the end
-                while (dockables.Count > 0 && dockables[dockables.Count - 1] is IProportionalDockSplitter)
+                while (dockables.Count > 0 && dockables[dockables.Count - 1] is ISplitter)
                 {
                     toRemove.Add(dockables[dockables.Count - 1]);
                     dockables.RemoveAt(dockables.Count - 1);
@@ -121,11 +121,11 @@ protected void CleanupOrphanedSplitters(IDock dock)
                 // Remove consecutive splitters - keep only the first one in each sequence
                 for (int i = 0; i < dockables.Count - 1; i++)
                 {
-                    if (dockables[i] is IProportionalDockSplitter)
+                    if (dockables[i] is ISplitter)
                     {
                         // Remove all consecutive splitters after this one
                         int j = i + 1;
-                        while (j < dockables.Count && dockables[j] is IProportionalDockSplitter)
+                        while (j < dockables.Count && dockables[j] is ISplitter)
                         {
                             toRemove.Add(dockables[j]);
                             j++;
@@ -147,10 +147,10 @@ protected void CleanupOrphanedSplitters(IDock dock)
         }
 
         // Update active dockable if it was a splitter that got removed
-        if (dock.ActiveDockable is IProportionalDockSplitter || 
+        if (dock.ActiveDockable is ISplitter || 
             (dock.ActiveDockable is not null && !dock.VisibleDockables.Contains(dock.ActiveDockable)))
         {
-            dock.ActiveDockable = dock.VisibleDockables?.FirstOrDefault(d => d is not IProportionalDockSplitter);
+            dock.ActiveDockable = dock.VisibleDockables?.FirstOrDefault(d => d is not ISplitter);
         }
     }
 
@@ -1254,7 +1254,7 @@ private void UpdateIsEmpty(IDock dock)
 
         var newIsEmpty = dock.VisibleDockables == null
                          || dock.VisibleDockables?.Count == 0
-                         || dock.VisibleDockables!.All(x => x is IDock { IsEmpty: true, IsCollapsable: true } or IProportionalDockSplitter);
+                         || dock.VisibleDockables!.All(x => x is IDock { IsEmpty: true, IsCollapsable: true } or ISplitter);
 
         if (oldIsEmpty != newIsEmpty)
         {

src/Dock.Model/FactoryBase.Init.cs

@@ -35,8 +35,19 @@ public virtual void InitLayout(IDockable layout)
             }
         }
 
+        // Auto-set ActiveDockable for RootDock if not already set
         if (layout is IRootDock rootDock)
         {
+            if (rootDock.ActiveDockable is null && rootDock.VisibleDockables is not null)
+            {
+                // Find first visible dockable that is not a splitter
+                var firstVisible = FindFirstVisibleDockable(rootDock);
+                if (firstVisible is not null)
+                {
+                    rootDock.ActiveDockable = firstVisible;
+                }
+            }
+
             if (rootDock.ShowWindows.CanExecute(null))
             {
                 rootDock.ShowWindows.Execute(null);
@@ -108,6 +119,43 @@ public virtual void InitDockable(IDockable dockable, IDockable? owner)
         OnDockableInit(dockable);
     }
 
+    /// <summary>
+    /// Finds the first visible dockable in the hierarchy that is not a splitter.
+    /// </summary>
+    /// <param name="dock">The dock to search in.</param>
+    /// <returns>The first visible dockable or null if none found.</returns>
+    private IDockable? FindFirstVisibleDockable(IDock dock)
+    {
+        if (dock.VisibleDockables is null)
+        {
+            return null;
+        }
+
+        // First look for direct visible dockables that are not splitters
+        foreach (var dockable in dock.VisibleDockables)
+        {
+            if (dockable is not ISplitter)
+            {
+                return dockable;
+            }
+        }
+
+        // If no direct dockables found, recursively search in child docks
+        foreach (var dockable in dock.VisibleDockables)
+        {
+            if (dockable is IDock childDock)
+            {
+                var result = FindFirstVisibleDockable(childDock);
+                if (result is not null)
+                {
+                    return result;
+                }
+            }
+        }
+
+        return null;
+    }
+
     private void InitDockables(IDockable dockable, IList<IDockable> dockables)
     {
         foreach (var child in dockables)