Adds VBE.MDIActiveChild method for getting the VBE's active MDI window even if it isn't the active VBE window.

Author: ThunderFrame

Rubberduck.VBEEditor/SafeComWrappers/Abstract/IVBE.cs

@@ -21,6 +21,7 @@ public interface IVBE : ISafeComWrapper, IEquatable<IVBE>
         IWindows Windows { get; }
 
         IHostApplication HostApplication();
+        IWindow ActiveMDIChild();
 
         bool IsInDesignMode { get; }
     }

Rubberduck.VBEEditor/SafeComWrappers/VB6/VBE.cs

@@ -109,6 +109,11 @@ public IHostApplication HostApplication()
             return null;
         }
 
+        public IWindow ActiveMDIChild()
+        {
+            throw new NotImplementedException();
+        }
+
         public bool IsInDesignMode
         {
             get { return VBProjects.All(project => project.Mode == EnvironmentMode.Design); }

Rubberduck.VBEEditor/SafeComWrappers/VBA/VBE.cs

@@ -3,6 +3,7 @@
 using System.IO;
 using System.Linq;
 using System.Runtime.InteropServices;
+using System.Text;
 using Rubberduck.VBEditor.Application;
 using Rubberduck.VBEditor.SafeComWrappers.Abstract;
 using Rubberduck.VBEditor.SafeComWrappers.MSForms;
@@ -279,6 +280,33 @@ public IHostApplication HostApplication()
             return null;
         }
 
+        /// <summary> Returns the topmost MDI child window. </summary>
+        public IWindow ActiveMDIChild()
+        {
+            const string mdiClientClass = "MDIClient";
+            const int maxCaptionLength = 512;
+
+            IntPtr mainWindow = (IntPtr)MainWindow.HWnd;
+
+            IntPtr mdiClient = NativeMethods.FindWindowEx(mainWindow, IntPtr.Zero, mdiClientClass, string.Empty);
+
+            IntPtr mdiChild = NativeMethods.GetTopWindow(mdiClient);
+            StringBuilder mdiChildCaption = new StringBuilder();
+            int captionLength = NativeMethods.GetWindowText(mdiChild, mdiChildCaption, maxCaptionLength);
+
+            if (captionLength > 0)
+            {
+                try
+                {
+                    return Windows[mdiChildCaption];
+                }
+                catch
+                {
+                }
+            }
+            return null;
+        }
+
         /// <summary> Returns whether the host supports unit tests.</summary>
         public bool HostSupportsUnitTests()
         {