Hello, Rick,

We understand how disruptive this issue is and we will do our best to introduce the fix as soon as possible. We also appreciate you sharing that version 2026.1.415 is working for you as a temporary workaround.

If you pay attention to the status of this feedback item, it is set to Planned, which means that we will include a fix for it in our next release version. This issue was prioritized and our dev team started working on it and a fix candidate is currently available. When all testing phases are finished we could be able to include in the next release.

We will update the item status with the released version where the fix will be available. So, make sure that you subscribe to this feedback item so you can receive automatic notifications when we change its status.

You're right that expectations for responsiveness and turnaround times are much higher today, and we take that seriously. We also recognize that you have alternatives available to you, and we do not take your continued patience or trust for granted. What I can assure you is that the issue has been reported to the appropriate engineering team, and the impact on customers is understood so we are currenly working to provide a new version where this issue is fixed. As soon as we have a confirmed timeline or update, we will communicate it.

Thank you again for your honest feedback and for continuing to engage with us while we work through this. We appreciate it.

Regards,
Nadya | Tech Support Engineer
Progress Telerik

Hello Satoshi,

First of all, I'd like to thank you for the time you took to provide detailed information about the problem and steps to reproduce it.

We were able to reproduce the issue at our end, and we have investigated it, thus the delayed response. The problem is caused by changes made at our end and also changes made by Microsoft in the latest version of Visual Studio 2026.

Before providing more details, I am glad to tell you we identified an improvement we can introduce with the next minor release of Telerik UI for WPF , which should resolve the problem. In the meantime, please use the 2026.1.415 version if possible, or reference individual controls packages from the latest version instead of the AllControls package.

Summary of findings

Changes in the Telerik NuGet package

With the latest release we have migrated from the NuGet packages built per target framework to an all encompassing AllControls package, that is multi-framework. In addition to that we have moved from building the package using nuspec file to building from a project, which led to dependencies being removed from the package itself.

Changes in Visual Studio

What Microsoft changed in Visual Studio:

1. The design-time features were decoupled from the main devenv process. This includes deprecation of legacy registry installers, which is the Microsoft.VisualStudio.IDE.ToolboxControlsInstaller.pkgdef file, which was responsible for merging third-party toolbox items via classical COM/Registry-based mechanisms. 

2. The XAML Designer operates in an isolated worker process. When it attempts to load the primary control assembly via the toolbox registration, it must also resolve all of its dependencies. If a dependency is missing from its expected probing path, the cross-process communication between devenv.exe and the designer process encounters a thread-blocking deadlock—causing the entire Visual Studio IDE to hang.

We believe that the latter is causing the issue as the main control assembly and all of its dependencies are no longer flattened directly into the same lib/ directory, and copied to the consuming project's output folder together.

Last, but not least, I'd like to tell you that I have updated your Telerik Points balance as an appreciation of your assistance in the efforts to make our products better.

Regards,
Nikola
Progress Telerik