Update: Kura has created a solution for this per this GitHub issue. You can look at the following post as a description as to how the registry works in regards to browsers.
Windows 10 users likely are familiar with its settings app by now which Microsoft wants to be a replacement for the Control Panel. Indeed, Microsoft took away some functionality of the Control Panel with the Fall Creators’ Update by diverting default app selections to within the settings.
This appears to not be too much of an issue at first glance. You can set including but not limited to a default music player, video player, and, of course, a web browser. The issue specifically is with trying to set an app that is not detected by settings. Windows 10 for better and worse has this dual identity where it wants to be a desktop and a mobile platform, and it definitely shows in various parts of the OS like with the settings and the Control Panel as I mentioned. It sort of makes sense sometimes because, for example, the Control Panel and settings have overlapping functionality. Although, it can make some wonder why Microsoft had to make the settings app at all. That is quite apparent here because the app only gives the option to select non-detected apps using the store. No option is given to select using File Explorer.
For settings, and the Control Panel to be fair, to detect a web browser, it has to be listed in the registry. This fortunately is an all right experience because only a few key-value pairs have to be added. Before starting, back up the registry, create a system restore point, etc. just in case the OS becomes negatively affected.
Now, type regedit into the start menu and press enter in order to open the registry. You will need administrator privileges to do this.
Navigate to Computer\HKEY_LOCAL_MACHINE\SOFTWARE\RegisteredApplications. You can also copy and paste this into the registry’s address bar. Listed here are all of the apps the registry has on file.
Add Sushi by right-clicking among the apps and selecting New > String. Name it Sushi, Sushi Browser, or anything else you can easily remember. Enter this as the value: SOFTWARE\Clients\StartMenuInternet\Sushi\Capabilities
The part of the value immedialy before Capabilities is the name of the app, once it is registered, that settings and the Control Panel will detect. I have it as Sushi, although you can have it as anything else.
Next, use the key’s value as an address and go as far as you can to that address. Said differently, navigate to Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\ and create Sushi by right-clicking within the directory and selecting New > Key. Repeat this for Capabilities while the Sushi key is selected.
Add the following strings to Capabilities: ApplicationDescription, ApplicationIcon, and ApplicationName. These appear to be optional but at least fill out the icon and the name. The value for the icon is <the executable’s location>,0. For example, C:\Users\Koe\AppData\Local\sushi\sushi.exe,0 is the value I have. Sushi Browser is set for the ApplicationName.
Capabilities will have three keys underneath it: FileAssociations, Startmenu, and URLAssociations. Create the keys as you have done before.
Starting with FileAssociations, create strings for each of the following: .htm, .html, .mht, .mhtml, .shtml, .xht, and .xhtml. These are the file types that settings and the Control Panel will recognize for Sushi. No data values are needed.
Create a string under Startmenu called StartMenuInternet, and set the value to Sushi Browser.
For UrlAssociations: ftp, http, and https, all with Sushi Browser as the values.
Click on the Sushi key again, and make a DefaultIcon key under it with C:\Users\<User Directory>\AppData\Local\sushi\sushi.exe,0 for the (Default) string.
Click on the Sushi key one more time, and create a key called shell under it. No data value is needed. Create a key called open under shell. Again, no data value is needed. Make a key called command under open. Enter in C:\Users\<User Directory>\AppData\Local\sushi\sushi.exe for the (Default) string.
Lastly, we need to create a class for Sushi Browser. The remaining steps essentially are a repeat of those immediately above.
Navigate to Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Classes\, and make a key for Sushi Browser. Make a DefaultIcon key under this. Enter in C:\Users\<User Directory>\AppData\Local\sushi\sushi.exe,0 for the (Default) string.
Click on the Sushi Browser key one more time, and create a key called shell under it. No data value is needed. Create a key called open under shell. Again, no data value is needed. Make a key called command under open. Enter in C:\Users\<User Directory>\AppData\Local\sushi\sushi.exe for the (Default) string.
Now, we can restart Windows in order for the changes to take effect. Log in, and open the settings. Navigate to the Defaults Apps, and click the space under Web browser. If the changes were successful, you should see Sushi Browser in your list of browsers. You should also see Sushi listed in the app, file type, and protocol selection menus.
Sushi Browser now can be set as a default through Windows 10’s settings. Because the settings and the Control Panel have similar functionality in this regard, this process also should work with Windows 7 and Windows 8.X. Other OS types such as Linux and Mac OS X supposedly do not have this issue, but ideally, Sushi should include functionality in its own settings to set itself as the default. We at least have this as a workaround solution until then.