Getting Windows Snap to Play with WPF Borderless Windows

In making the Dragablz library I quickly realised I needed be able to push the tabs higher up the window as we see in Chrome, and also, to achieve the IE affect, I really needed Window transparency. Therefore I introduced DragablzWindow.


The easiest way to do the above is set the WindowStyle to None. But this has immediate draw backs: no dragging and resizing. Getting the dragging up and running is pretty easy and well illustrated on the web: 

MouseLeftButtonDown += (s, e) => DragMove(); 

But this only partially works with Windows Snap. The Window will snap; left, right, top/maximised. But you cant drag the maximised window down to restore it.

Snap Attack

So I had to blend some aspects of WPF and WinApi to achieve the full effect.

In the XAML template for DragablzWindow I placed a Thumb control behind the content. Note how the hit test is off:

<Thumb Style="{StaticResource InvisibleThumbStyle}"  
<ContentPresenter Margin="4"/> 

The hit test is off so it doesn't interfere with our DragMove. Until the Window becomes maximised. Once the Window becomes maximised we enable the thumb and listen to it's drag delta. Monitoring the drag delta is where we perform our trick, sending a Windows message to restart the drag as usual, handing back off the Thumb, to an API instigated drag. Basically a slight re-working of what happens inside Windows.DragMove:

private void WindowMoveThumbOnDragDelta(object sender, DragDeltaEventArgs dragDeltaEventArgs)  
    if (WindowState != WindowState.Maximized ||  
        (!(Math.Abs(dragDeltaEventArgs.HorizontalChange) > 2) &&  
         !(Math.Abs(dragDeltaEventArgs.VerticalChange) > 2))) return;  
    WindowState = WindowState.Normal;  
    Native.SendMessage(CriticalHandle, WindowMessage.WM_LBUTTONUP, IntPtr.Zero, IntPtr.Zero);  
    Native.SendMessage(CriticalHandle, WindowMessage.WM_SYSCOMMAND, (IntPtr)SystemCommand.SC_MOUSEMOVE, IntPtr.Zero);  

Try this on for Resize

To make the Window resizable we get back to the WPF side of things, using another Thumb, this time laid over the content (instead of under). Applying a custom clip, we can make the thumb only hit-able around the border of the Window, leaving the behavior of the remaining content intact:

protected override void OnRenderSizeChanged(SizeChangedInfo sizeInfo)  
    var resizeThumb = GetTemplateChild(WindowResizeThumbPartName) as Thumb;  
    if (resizeThumb != null)  
        var outerRectangleGeometry = new RectangleGeometry(new Rect(sizeInfo.NewSize));  
        var innerRectangleGeometry =  
            new RectangleGeometry(new Rect(ResizeMargin, ResizeMargin, sizeInfo.NewSize.Width - ResizeMargin * 2, sizeInfo.NewSize.Height - ResizeMargin*2));  
        resizeThumb.Clip = new CombinedGeometry(GeometryCombineMode.Exclude, outerRectangleGeometry,  

We must now handle the sizing manually, based on how the user drags the thumb around. It's worth seeing the source code to see how that's handled.

The Result

A Window which supports:

  • Transparancy
  • Dragging
  • Resizing
  • Snapping
  • And, all of the cool tab features of Dragablz!


Source Code

The DragablzWindow is part of the Dragablz project on GitHub. And the style is here.