Windows 8 and 10 users may encounter something appearing to be a program called “TWINUI” when opening links and files from within a program such as a web browser. TWINUI is often mistaken for an email client or PDF reader because it most often appear when a user follows an email link or tries to open a PDF in web browsers like Firefox. However, it’s really a much less concise and more important thing than a just a single program.
TWinUI — or Tablet Windows User Interface — is included in Windows 8 and newer. It’s not a program on its own, but rather a system library that is part of the Windows Shell that provides a set of functionality — including starting applications to Windows itself and other programs. TWINUI is often mistaken for a built‐in PDF viewer or email client in forums and discussion online. This is most likely because most users will only encounter the name “TWINUI” when they either open a PDF file from their browsers or click on a mailto:, call:, or other protocol link recognized by the Windows app launcher.
The name TWinUI is also often misunderstood to literally mean “Twin UI”, referring to the duality of the modern and the classic Windows desktop living side by side. Although Windows has not properly named nor documented a name for this library, it’s pretty safe to assume from function names found in the twinui.dll library that it refers to Tablet Windows UI. It could possibly be Touch Windows UI, but there are a lot more functions throughout Windows containing references to TWINUI that name things “Tablet” rather than “Touch”. It looks like Microsoft forgot to set a friendly name for their new application launcher system, which means it inherited the TWINUI name from the parent library. Microsoft apparently never got around to updating the name or didn’t think users would ever actually see it anywhere in Windows.
Keep in mind that Microsoft weren’t entirely sure what they were going to call “the new Windows experience” in Windows 8 themselves up until weeks before the release. They named the new desktop shell “Metro” just before launch and were forced to change it intermittently to “Modern Windows” because of a trademark dispute. For Windows 10, what was once the new Windows experience is simply called the Windows Shell presented in either desktop or immersive (tablet) mode.
There is a bug in Windows 10 that shows for some users where they can’t change their system’s default programs in the Settings app. All their defaults have been reset to show “TWINUI”. This simply means that they have no default programs set, usually because of System Registry corruption, and they’re getting the same bad fallback name as surface in Firefox. The problem can be resolved by restoring the registry from a backup before the corruption occurred.
TWINUI is sometimes deferred to another system which ha a proper name, “Pick an app”, provided by OpenWith. OpenWith.exe is another part of the Windows files and application launcher system. When users see this rather than TWINUI, it means that there are multiple programs that have registered handlers for the same file type or link protocol and that the user must make a choice or pick a default to avoid choosing every time in the future.