Summary answer so far: Forms and controls only open in the designer if csproj.user file exists.

The problem story:

I have 2 controls that both inherit from a base control. However one is missing the control icon in the solution explorer

The good control source starts with

public partial class UniTabMaterialsControl : UnitabBaseControl
{
    public UniTabMaterialsControl()
    {
        InitializeComponent();
    }

The bad control source starts with

public partial class UniTabMaterialsControl : UnitabBaseControl
{
     
    public UniTabMaterialsControl()
    {
        InitializeComponent();
    }

The base control class is

public class UnitabBaseControl : UserControl
{
}

I have tried closing and re-opening Visual Studio

[Update]

I added a new user control and set it to also inherit from UnitabBaseControl , strangely the behaviour corrected.

I was then able to delete the new control and maintain correct behaviour

Git shows that there has been a change in UnitabBaseControl.cs but I do not see what changed other than the colour of the UserControl text

I then made a fresh clone of the whole solution. This time the incorrect behaviour showed on all the controls that inherited from UnitabBaseControl

[Update]

Now I suffer the same issue with forms

using System.Windows.Forms;

namespace SBD.VivSnap.UI
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
    }
}

and

using System.Windows.Forms;

namespace SBD.VivSnap.UI
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
    }
}

[Update]

I neglected to mention that my project was a library project referred to by the start up project.

I deleted and then re-added the library project reference in the start up project. Then all the forms and controls appeared correctly.

I could not find any changes in Git to indicate why the solution now worked.

Since then I have noticed the problem in my main project

[Update]

It turns out that my project is in .sdk format even though it is framework 4.7.2 I am thinking this may have something to do with it.

I am using Framework 4.7.2 with an sdk project

Project Sdk="Microsoft.NET.Sdk.WindowsDesktop"

I checked out the code onto a different computer running VS2019 16.1.4 and it builds ok.

I am using 16.11.4

[Update] Now I have updated to 16.11.5 and cloned again but still have the problem.

[Update] The project file is

<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
  <PropertyGroup>
    <TargetFramework>net472</TargetFramework>
    <OutputType>WinExe</OutputType>
    <AssemblyName>Main</AssemblyName>
    <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\</SolutionDir>
    <RestorePackages>true</RestorePackages>
    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>
  <PropertyGroup>
    <StartupObject>SBD.VivSnap.Main.Program</StartupObject>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.CSharp" Version="4.7.0" />
    <PackageReference Include="Microsoft.Data.SqlClient">
      <Version>2.0.1</Version>
    </PackageReference>
    <PackageReference Include="Microsoft.EntityFrameworkCore">
      <Version>3.1.12</Version>
    </PackageReference>
    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer">
      <Version>3.1.12</Version>
    </PackageReference>
    <PackageReference Include="NuGet.Build.Tasks.Pack" Version="5.7.0">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>
    <PackageReference Include="System.ComponentModel.Annotations" Version="5.0.0" />
    <PackageReference Include="System.Data.DataSetExtensions" Version="4.5.0" />
  </ItemGroup>
  <ItemGroup>
    <ProjectReference Include="..\SnapInUI\SBD.VivSnap.UI.csproj" />
  </ItemGroup>
  <ItemGroup>
    <Compile Update="Properties\Settings.Designer.cs">
      <DesignTimeSharedInput>True</DesignTimeSharedInput>
      <AutoGen>True</AutoGen>
      <DependentUpon>Settings.settings</DependentUpon>
    </Compile>
  </ItemGroup>
  <ItemGroup>
    <None Update="Properties\Settings.settings">
      <Generator>SettingsSingleFileGenerator</Generator>
      <LastGenOutput>Settings.Designer.cs</LastGenOutput>
    </None>
  </ItemGroup>
</Project>

The csproj.user file is in my answer.