runtime: Reading configuration settings fails with System.UriFormatException: 'Invalid URI: The hostname could not be parsed.'

Description

Observed while using the new Windows Forms designer to design a migrated app from net461 to net5.0. This doesn’t appear to be a problem while running an app, but it is a problem when the code is executed in the Windows Forms designers (which is now run out of proc).

at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind)
at System.Uri..ctor(String uriString)
at System.Configuration.ClientConfigPaths..ctor(String exePath, Boolean includeUserConfig)
at System.Configuration.ClientConfigPaths.GetPaths(String exePath, Boolean includeUserConfig)
at System.Configuration.Internal.ConfigurationManagerInternal.System.Configuration.Internal.IConfigurationManagerInternal.get_ExeProductName()
at System.Configuration.ApplicationSettingsBase.get_Initializer()
at System.Configuration.ApplicationSettingsBase.CreateSetting(PropertyInfo propertyInfo)
at System.Configuration.ApplicationSettingsBase.EnsureInitialized()
at System.Configuration.ApplicationSettingsBase.get_Properties()
at System.Configuration.SettingsBase.GetPropertyValueByName(String propertyName)
at System.Configuration.SettingsBase.get_Item(String propertyName)
at System.Configuration.ApplicationSettingsBase.GetPropertyValue(String propertyName)
at System.Configuration.ApplicationSettingsBase.get_Item(String propertyName)
at GitCommands.Properties.Settings.get_IsPortable() in D:\Development\gitextensions2\GitCommands\Properties\Settings.Designer.cs:line 31

When I debugged in, for some strange reason AppDomain.CurrentDomain.BaseDirectory was returning \\?\C:\Users\igveliko\AppData\Local\Temp\WinFormsCache\exm5a4ew.gwt\

However when try run the same code locally AppDomain.CurrentDomain.BaseDirectory returns path without leading \\?\:

In essence this, what I think, is happening:

string baseDirectory = @"\\?\C:\Users\igveliko\AppData\Local\Temp\WinFormsCache\exm5a4ew.gwt\";
string manifestModuleName = @"DesignToolsServer.dll";
string configBasePath = Path.Combine(baseDirectory, manifestModuleName);
Uri uri = new Uri(configBasePath);
Console.WriteLine(uri.LocalPath);

Repro steps

1. git clone -v --recurse-submodules --depth 1 --no-single-branch --progress "https://github.com/gitextensions/gitextensions.git" "<path>/gitextensions"
2. dotnet restore
3. Open GitExtensions.sln in VS
4. Ignore Wix warning, trust all repos
5. Navigate to AppSettings.IsPortable() and remove IsDesignMode

   diff --git a/GitCommands/Settings/AppSettings.cs b/GitCommands/Settings/AppSettings.cs
   
   -        public static bool IsPortable() => !IsDesignMode && Properties.Settings.Default.IsPortable;
   +        public static bool IsPortable() => Properties.Settings.Default.IsPortable;
   

6. Compile
7. Open FormAbout in designer

Configuration

• .NET 5.0.6
• .NET: 6.0.0-rc.1.21401.1

Regression?

¯\_(ツ)_/¯ I struggle to tell what is the cause of this strange behaviour