On Windows 10 (1803), all applications lost touch or stylus if a WPF transparent window covers on them

Question

If I create a new WPF application with a simple empty window like the code shown below, I find that all applications which are covered by the WPF app lost touch or stylus reaction. This can only be reproduced when Windows 10 is upgraded to 1803 (10.0.17134.0).

<Window x:Class="TheWPFCoveringWindow.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        WindowStyle="None" WindowState="Maximized"
        AllowsTransparency="True" Background="Transparent"
        Topmost="True">
    <Button Content="Test" Width="200" Height="100" />
</Window>

I wrote another WPF application to find out what happened. So I add a StylusDown event to the Window like the code shown below:

// This code is in another WPF application.
private void OnStylusDown(object sender, StylusDownEventArgs e)
{
    // Set a breakpoint here.
}

But the breakpoint never reached until I closed the transparent WPF window which is on top.

I pushed the very simple code to GitHub: dotnet-campus/TouchIssueOnWindows10.0.17134. Cloning it might help a little.

Why does this happen and how to solve it? Any reply is appreciated.

Answer 1

Updated

Microsoft has fixed this issue in .NET Framework August 2018 Preview of Quality Rollup.

• August 30, 2018—KB4346783 (OS Build 17134.254)

Addresses an issue where touch and mouse events were handled differently in Windows Presentation Foundation (WPF) applications that have a transparent overlay window.

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Original

After a whole week's debugging, I finally find out the solution.

Just add a ResizeMode="NoResize" property for the Window as the code shown below:

<Window x:Class="TheWPFCoveringWindow.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        WindowStyle="None" WindowState="Maximized"
        AllowsTransparency="True" ResizeMode="NoResize"
        Background="Transparent" Topmost="True">
    <Button Content="Test" Width="200" Height="100" />
</Window>

@lindexi has posted this issue and this solution into his post. If you want more information about this issue, read win10 17025 touch bug - lindexi for more details. (This post is written in multiple languages, so you'll miss nothing even if you ignore the unknown characters.)

Actually, I still can't figure out why this property helps.

Could anyone explain the reason for this issue?