C#: Make editor create NuGet fallback folder for Godot packages

Main benefits:
- Projects can be built offline. Previously you needed internet
  access the first time building to download the packages.
- Changes to packages like Godot.NET.Sdk can be easily tested
  before publishing. This was already possible but required
  too many manual steps.
- First time builds are a bit faster, as the Sdk package doesn't
  need to be downloaded. In practice, the package is very small
  so it makes little difference.

Bumped Godot.NET.Sdk to 4.0.0-dev3 in order to enable the
recent changes regarding '.mono/' -> '.godot/mono/'.

13 files changed, 472 insertions, 6 deletions

diff --git a/modules/mono/SCsub b/modules/mono/SCsub
index e8f3174a0a..3b94949470 100644
--- a/modules/mono/SCsub
+++ b/modules/mono/SCsub
@@ -40,6 +40,11 @@ if env_mono["tools"] and env_mono["mono_glue"] and env_mono["build_cil"]:
 
     godot_tools_build.build(env_mono, api_sln_cmd)
 
+    # Build Godot.NET.Sdk
+    import build_scripts.godot_net_sdk_build as godot_net_sdk_build
+
+    godot_net_sdk_build.build(env_mono)
+
 # Add sources
 
 env_mono.add_source_files(env.modules_sources, "*.cpp")
diff --git a/modules/mono/build_scripts/godot_net_sdk_build.py b/modules/mono/build_scripts/godot_net_sdk_build.py
new file mode 100644
index 0000000000..3bfba0f0f6
--- /dev/null
+++ b/modules/mono/build_scripts/godot_net_sdk_build.py
@@ -0,0 +1,45 @@
+# Build Godot.NET.Sdk solution
+
+import os
+
+from SCons.Script import Dir
+
+
+def build_godot_net_sdk(source, target, env):
+    # source and target elements are of type SCons.Node.FS.File, hence why we convert them to str
+
+    module_dir = env["module_dir"]
+
+    solution_path = os.path.join(module_dir, "editor/Godot.NET.Sdk/Godot.NET.Sdk.sln")
+    build_config = "Release"
+
+    from .solution_builder import build_solution
+
+    extra_msbuild_args = ["/p:GodotPlatform=" + env["platform"]]
+
+    build_solution(env, solution_path, build_config, extra_msbuild_args)
+    # No need to copy targets. The Godot.NET.Sdk csproj takes care of copying them.
+
+
+def build(env_mono):
+    assert env_mono["tools"]
+
+    output_dir = Dir("#bin").abspath
+    editor_tools_dir = os.path.join(output_dir, "GodotSharp", "Tools")
+    nupkgs_dir = os.path.join(editor_tools_dir, "nupkgs")
+
+    module_dir = os.getcwd()
+
+    package_version_file = os.path.join(
+        module_dir, "editor", "Godot.NET.Sdk", "Godot.NET.Sdk", "Godot.NET.Sdk_PackageVersion.txt"
+    )
+
+    with open(package_version_file, mode="r") as f:
+        version = f.read().strip()
+
+    target_filenames = ["Godot.NET.Sdk.%s.nupkg" % version]
+
+    targets = [os.path.join(nupkgs_dir, filename) for filename in target_filenames]
+
+    cmd = env_mono.CommandNoCache(targets, [], build_godot_net_sdk, module_dir=module_dir)
+    env_mono.AlwaysBuild(cmd)
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj
index 86a0a4393e..8304d9e321 100644
--- a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk.csproj
@@ -1,13 +1,13 @@
 <Project Sdk="Microsoft.Build.NoTargets/2.0.1">
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
+    <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
 
     <Description>MSBuild .NET Sdk for Godot projects.</Description>
     <Authors>Godot Engine contributors</Authors>
 
     <PackageId>Godot.NET.Sdk</PackageId>
     <Version>4.0.0</Version>
-    <PackageVersion>4.0.0-dev2</PackageVersion>
     <PackageProjectUrl>https://github.com/godotengine/godot/tree/master/modules/mono/editor/Godot.NET.Sdk</PackageProjectUrl>
     <PackageType>MSBuildSdk</PackageType>
     <PackageTags>MSBuildSdk</PackageTags>
@@ -19,7 +19,13 @@
     <GenerateNuspecDependsOn>$(GenerateNuspecDependsOn);SetNuSpecProperties</GenerateNuspecDependsOn>
   </PropertyGroup>
 
-  <Target Name="SetNuSpecProperties" Condition=" Exists('$(NuspecFile)') ">
+  <Target Name="ReadGodotNETSdkVersion" BeforeTargets="BeforeBuild;BeforeRebuild;CoreCompile">
+    <PropertyGroup>
+      <PackageVersion>$([System.IO.File]::ReadAllText('$(ProjectDir)Godot.NET.Sdk_PackageVersion.txt').Trim())</PackageVersion>
+    </PropertyGroup>
+  </Target>
+
+  <Target Name="SetNuSpecProperties" Condition=" Exists('$(NuspecFile)') " DependsOnTargets="ReadGodotNETSdkVersion">
     <PropertyGroup>
       <NuspecProperties>
         id=$(PackageId);
@@ -32,4 +38,13 @@
       </NuspecProperties>
     </PropertyGroup>
   </Target>
+
+  <Target Name="CopyNupkgToSConsOutputDir" AfterTargets="Pack">
+    <PropertyGroup>
+      <GodotSourceRootPath>$(SolutionDir)\..\..\..\..\</GodotSourceRootPath>
+      <GodotOutputDataDir>$(GodotSourceRootPath)\bin\GodotSharp\</GodotOutputDataDir>
+    </PropertyGroup>
+    <Copy SourceFiles="$(OutputPath)$(PackageId).$(PackageVersion).nupkg"
+          DestinationFolder="$(GodotOutputDataDir)Tools\nupkgs\" />
+  </Target>
 </Project>
diff --git a/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk_PackageVersion.txt b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk_PackageVersion.txt
new file mode 100644
index 0000000000..34749489b9
--- /dev/null
+++ b/modules/mono/editor/Godot.NET.Sdk/Godot.NET.Sdk/Godot.NET.Sdk_PackageVersion.txt
@@ -0,0 +1 @@
+4.0.0-dev3
diff --git a/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/GodotTools.ProjectEditor.csproj b/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/GodotTools.ProjectEditor.csproj
index e4d6b2e010..37123ba2b2 100644
--- a/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/GodotTools.ProjectEditor.csproj
+++ b/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/GodotTools.ProjectEditor.csproj
@@ -10,6 +10,7 @@
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\GodotTools.Core\GodotTools.Core.csproj" />
+    <ProjectReference Include="..\GodotTools.Shared\GodotTools.Shared.csproj" />
   </ItemGroup>
   <!--
   The Microsoft.Build.Runtime package is too problematic so we create a MSBuild.exe stub. The workaround described
diff --git a/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/ProjectGenerator.cs b/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/ProjectGenerator.cs
index 01d7c99662..7d49d251dd 100644
--- a/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/ProjectGenerator.cs
+++ b/modules/mono/editor/GodotTools/GodotTools.ProjectEditor/ProjectGenerator.cs
@@ -3,14 +3,13 @@ using System.IO;
 using System.Text;
 using Microsoft.Build.Construction;
 using Microsoft.Build.Evaluation;
+using GodotTools.Shared;
 
 namespace GodotTools.ProjectEditor
 {
     public static class ProjectGenerator
     {
-        public const string GodotSdkVersionToUse = "4.0.0-dev2";
-
-        public static string GodotSdkAttrValue => $"Godot.NET.Sdk/{GodotSdkVersionToUse}";
+        public static string GodotSdkAttrValue => $"Godot.NET.Sdk/{GeneratedGodotNupkgsVersions.GodotNETSdk}";
 
         public static ProjectRootElement GenGameProject(string name)
         {
diff --git a/modules/mono/editor/GodotTools/GodotTools.Shared/GenerateGodotNupkgsVersions.targets b/modules/mono/editor/GodotTools/GodotTools.Shared/GenerateGodotNupkgsVersions.targets
new file mode 100644
index 0000000000..1d382dcb43
--- /dev/null
+++ b/modules/mono/editor/GodotTools/GodotTools.Shared/GenerateGodotNupkgsVersions.targets
@@ -0,0 +1,36 @@
+<Project>
+  <!-- Generate C# file with the version of all the nupkgs bundled with Godot -->
+
+  <Target Name="SetPropertiesForGenerateGodotNupkgsVersions">
+    <PropertyGroup>
+      <GodotNETSdkPackageVersionFile>$(SolutionDir)..\Godot.NET.Sdk\Godot.NET.Sdk\Godot.NET.Sdk_PackageVersion.txt</GodotNETSdkPackageVersionFile>
+      <GeneratedGodotNupkgsVersionsFile>$(IntermediateOutputPath)GodotNupkgsVersions.g.cs</GeneratedGodotNupkgsVersionsFile>
+    </PropertyGroup>
+  </Target>
+
+  <Target Name="GenerateGodotNupkgsVersionsFile"
+          DependsOnTargets="PrepareForBuild;_GenerateGodotNupkgsVersionsFile"
+          BeforeTargets="BeforeCompile;CoreCompile">
+    <ItemGroup>
+      <Compile Include="$(GeneratedGodotNupkgsVersionsFile)" />
+      <FileWrites Include="$(GeneratedGodotNupkgsVersionsFile)" />
+    </ItemGroup>
+  </Target>
+  <Target Name="_GenerateGodotNupkgsVersionsFile"
+          DependsOnTargets="SetPropertiesForGenerateGodotNupkgsVersions"
+          Inputs="$(MSBuildProjectFile);@(GodotNETSdkPackageVersionFile)"
+          Outputs="$(GeneratedGodotNupkgsVersionsFile)">
+    <PropertyGroup>
+      <GenerateGodotNupkgsVersionsCode><![CDATA[
+namespace $(RootNamespace) {
+    public class GeneratedGodotNupkgsVersions {
+        public const string GodotNETSdk = "$([System.IO.File]::ReadAllText('$(GodotNETSdkPackageVersionFile)').Trim())"%3b
+    }
+}
+]]></GenerateGodotNupkgsVersionsCode>
+    </PropertyGroup>
+    <WriteLinesToFile Lines="$(GenerateGodotNupkgsVersionsCode)"
+                      File="$(GeneratedGodotNupkgsVersionsFile)"
+                      Overwrite="True" WriteOnlyWhenDifferent="True" />
+  </Target>
+</Project>
diff --git a/modules/mono/editor/GodotTools/GodotTools.Shared/GodotTools.Shared.csproj b/modules/mono/editor/GodotTools/GodotTools.Shared/GodotTools.Shared.csproj
new file mode 100644
index 0000000000..3bc1698c15
--- /dev/null
+++ b/modules/mono/editor/GodotTools/GodotTools.Shared/GodotTools.Shared.csproj
@@ -0,0 +1,6 @@
+<Project Sdk="Microsoft.NET.Sdk">
+    <PropertyGroup>
+        <TargetFramework>netstandard2.0</TargetFramework>
+    </PropertyGroup>
+    <Import Project="GenerateGodotNupkgsVersions.targets" />
+</Project>
diff --git a/modules/mono/editor/GodotTools/GodotTools.sln b/modules/mono/editor/GodotTools/GodotTools.sln
index ba5379e562..d3107a69db 100644
--- a/modules/mono/editor/GodotTools/GodotTools.sln
+++ b/modules/mono/editor/GodotTools/GodotTools.sln
@@ -13,6 +13,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "GodotTools.IdeMessaging", "
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "GodotTools.OpenVisualStudio", "GodotTools.OpenVisualStudio\GodotTools.OpenVisualStudio.csproj", "{EAFFF236-FA96-4A4D-BD23-0E51EF988277}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "GodotTools.Shared", "GodotTools.Shared\GodotTools.Shared.csproj", "{2758FFAF-8237-4CF2-B569-66BF8B3587BB}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -43,5 +45,9 @@ Global
 		{EAFFF236-FA96-4A4D-BD23-0E51EF988277}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{EAFFF236-FA96-4A4D-BD23-0E51EF988277}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{EAFFF236-FA96-4A4D-BD23-0E51EF988277}.Release|Any CPU.Build.0 = Release|Any CPU
+		{2758FFAF-8237-4CF2-B569-66BF8B3587BB}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{2758FFAF-8237-4CF2-B569-66BF8B3587BB}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{2758FFAF-8237-4CF2-B569-66BF8B3587BB}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{2758FFAF-8237-4CF2-B569-66BF8B3587BB}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 EndGlobal
diff --git a/modules/mono/editor/GodotTools/GodotTools/Build/BuildManager.cs b/modules/mono/editor/GodotTools/GodotTools/Build/BuildManager.cs
index 9b512d90f5..b96b0c8175 100644
--- a/modules/mono/editor/GodotTools/GodotTools/Build/BuildManager.cs
+++ b/modules/mono/editor/GodotTools/GodotTools/Build/BuildManager.cs
@@ -7,7 +7,6 @@ using JetBrains.Annotations;
 using static GodotTools.Internals.Globals;
 using File = GodotTools.Utils.File;
 using OS = GodotTools.Utils.OS;
-using Path = System.IO.Path;
 
 namespace GodotTools.Build
 {
@@ -209,6 +208,16 @@ namespace GodotTools.Build
             if (!File.Exists(GodotSharpDirs.ProjectSlnPath))
                 return true; // No solution to build
 
+            try
+            {
+                // Make sure our packages are added to the fallback folder
+                NuGetUtils.AddBundledPackagesToFallbackFolder(NuGetUtils.GodotFallbackFolderPath);
+            }
+            catch (Exception e)
+            {
+                Godot.GD.PushError("Failed to setup Godot NuGet Offline Packages: " + e.Message);
+            }
+
             GenerateEditorScriptMetadata();
 
             if (GodotSharpEditor.Instance.SkipBuildBeforePlaying)
diff --git a/modules/mono/editor/GodotTools/GodotTools/Build/MSBuildPanel.cs b/modules/mono/editor/GodotTools/GodotTools/Build/MSBuildPanel.cs
index c22dce005c..708ec73454 100644
--- a/modules/mono/editor/GodotTools/GodotTools/Build/MSBuildPanel.cs
+++ b/modules/mono/editor/GodotTools/GodotTools/Build/MSBuildPanel.cs
@@ -33,6 +33,16 @@ namespace GodotTools.Build
             if (!File.Exists(GodotSharpDirs.ProjectSlnPath))
                 return; // No solution to build
 
+            try
+            {
+                // Make sure our packages are added to the fallback folder
+                NuGetUtils.AddBundledPackagesToFallbackFolder(NuGetUtils.GodotFallbackFolderPath);
+            }
+            catch (Exception e)
+            {
+                GD.PushError("Failed to setup Godot NuGet Offline Packages: " + e.Message);
+            }
+
             BuildManager.GenerateEditorScriptMetadata();
 
             if (!BuildManager.BuildProjectBlocking("Debug"))
@@ -54,6 +64,16 @@ namespace GodotTools.Build
             if (!File.Exists(GodotSharpDirs.ProjectSlnPath))
                 return; // No solution to build
 
+            try
+            {
+                // Make sure our packages are added to the fallback folder
+                NuGetUtils.AddBundledPackagesToFallbackFolder(NuGetUtils.GodotFallbackFolderPath);
+            }
+            catch (Exception e)
+            {
+                GD.PushError("Failed to setup Godot NuGet Offline Packages: " + e.Message);
+            }
+
             BuildManager.GenerateEditorScriptMetadata();
 
             if (!BuildManager.BuildProjectBlocking("Debug", targets: new[] {"Rebuild"}))
diff --git a/modules/mono/editor/GodotTools/GodotTools/Build/NuGetUtils.cs b/modules/mono/editor/GodotTools/GodotTools/Build/NuGetUtils.cs
new file mode 100644
index 0000000000..793ef7fd71
--- /dev/null
+++ b/modules/mono/editor/GodotTools/GodotTools/Build/NuGetUtils.cs
@@ -0,0 +1,296 @@
+using System;
+using System.IO;
+using System.IO.Compression;
+using System.Linq;
+using System.Security.Cryptography;
+using System.Text;
+using System.Xml;
+using Godot;
+using GodotTools.Internals;
+using GodotTools.Shared;
+using Directory = GodotTools.Utils.Directory;
+using Environment = System.Environment;
+using File = GodotTools.Utils.File;
+
+namespace GodotTools.Build
+{
+    public static class NuGetUtils
+    {
+        public const string GodotFallbackFolderName = "Godot Offline Packages";
+
+        public static string GodotFallbackFolderPath
+            => Path.Combine(GodotSharpDirs.MonoUserDir, "GodotNuGetFallbackFolder");
+
+        private static void AddFallbackFolderToNuGetConfig(string nuGetConfigPath, string name, string path)
+        {
+            var xmlDoc = new XmlDocument();
+            xmlDoc.Load(nuGetConfigPath);
+
+            const string nuGetConfigRootName = "configuration";
+
+            var rootNode = xmlDoc.DocumentElement;
+
+            if (rootNode == null)
+            {
+                // No root node, create it
+                rootNode = xmlDoc.CreateElement(nuGetConfigRootName);
+                xmlDoc.AppendChild(rootNode);
+
+                // Since this can be considered pretty much a new NuGet.Config, add the default nuget.org source as well
+                XmlElement nugetOrgSourceEntry = xmlDoc.CreateElement("add");
+                nugetOrgSourceEntry.Attributes.Append(xmlDoc.CreateAttribute("key")).Value = "nuget.org";
+                nugetOrgSourceEntry.Attributes.Append(xmlDoc.CreateAttribute("value")).Value = "https://api.nuget.org/v3/index.json";
+                nugetOrgSourceEntry.Attributes.Append(xmlDoc.CreateAttribute("protocolVersion")).Value = "3";
+                rootNode.AppendChild(xmlDoc.CreateElement("packageSources")).AppendChild(nugetOrgSourceEntry);
+            }
+            else
+            {
+                // Check that the root node is the expected one
+                if (rootNode.Name != nuGetConfigRootName)
+                    throw new Exception("Invalid root Xml node for NuGet.Config. " +
+                                        $"Expected '{nuGetConfigRootName}' got '{rootNode.Name}'.");
+            }
+
+            var fallbackFoldersNode = rootNode["fallbackPackageFolders"] ??
+                                      rootNode.AppendChild(xmlDoc.CreateElement("fallbackPackageFolders"));
+
+            // Check if it already has our fallback package folder
+            for (var xmlNode = fallbackFoldersNode.FirstChild; xmlNode != null; xmlNode = xmlNode.NextSibling)
+            {
+                if (xmlNode.NodeType != XmlNodeType.Element)
+                    continue;
+
+                var xmlElement = (XmlElement)xmlNode;
+                if (xmlElement.Name == "add" &&
+                    xmlElement.Attributes["key"]?.Value == name &&
+                    xmlElement.Attributes["value"]?.Value == path)
+                {
+                    return;
+                }
+            }
+
+            XmlElement newEntry = xmlDoc.CreateElement("add");
+            newEntry.Attributes.Append(xmlDoc.CreateAttribute("key")).Value = name;
+            newEntry.Attributes.Append(xmlDoc.CreateAttribute("value")).Value = path;
+
+            fallbackFoldersNode.AppendChild(newEntry);
+
+            xmlDoc.Save(nuGetConfigPath);
+        }
+
+        /// <summary>
+        /// Returns all the paths where the user NuGet.Config files can be found.
+        /// Does not determine whether the returned files exist or not.
+        /// </summary>
+        private static string[] GetAllUserNuGetConfigFilePaths()
+        {
+            // Where to find 'NuGet/NuGet.Config':
+            //
+            // - Mono/.NETFramework (standalone NuGet):
+            //     Uses Environment.SpecialFolder.ApplicationData
+            //     - Windows: '%APPDATA%'
+            //     - Linux/macOS: '$HOME/.config'
+            // - CoreCLR (dotnet CLI NuGet):
+            //     - Windows: '%APPDATA%'
+            //     - Linux/macOS: '$DOTNET_CLI_HOME/.nuget' otherwise '$HOME/.nuget'
+
+            string applicationData = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData);
+
+            if (Utils.OS.IsWindows)
+            {
+                // %APPDATA% for both
+                return new[] {Path.Combine(applicationData, "NuGet", "NuGet.Config")};
+            }
+
+            var paths = new string[2];
+
+            // CoreCLR (dotnet CLI NuGet)
+
+            string dotnetCliHome = Environment.GetEnvironmentVariable("DOTNET_CLI_HOME");
+            if (!string.IsNullOrEmpty(dotnetCliHome))
+            {
+                paths[0] = Path.Combine(dotnetCliHome, ".nuget", "NuGet", "NuGet.Config");
+            }
+            else
+            {
+                string home = Environment.GetEnvironmentVariable("HOME");
+                if (string.IsNullOrEmpty(home))
+                    throw new InvalidOperationException("Required environment variable 'HOME' is not set.");
+                paths[0] = Path.Combine(home, ".nuget", "NuGet", "NuGet.Config");
+            }
+
+            // Mono/.NETFramework (standalone NuGet)
+
+            // ApplicationData is $HOME/.config on Linux/macOS
+            paths[1] = Path.Combine(applicationData, "NuGet", "NuGet.Config");
+
+            return paths;
+        }
+
+        // nupkg extraction
+        //
+        // Exclude: (NuGet.Client -> NuGet.Packaging.PackageHelper.ExcludePaths)
+        // package/
+        // _rels/
+        // [Content_Types].xml
+        //
+        // Don't ignore files that begin with a dot (.)
+        //
+        // The nuspec is not lower case inside the nupkg but must be made lower case when extracted.
+
+        /// <summary>
+        /// Adds the specified fallback folder to the user NuGet.Config files,
+        /// for both standalone NuGet (Mono/.NETFramework) and dotnet CLI NuGet.
+        /// </summary>
+        public static void AddFallbackFolderToUserNuGetConfigs(string name, string path)
+        {
+            foreach (string nuGetConfigPath in GetAllUserNuGetConfigFilePaths())
+            {
+                if (!System.IO.File.Exists(nuGetConfigPath))
+                {
+                    // It doesn't exist, so we create a default one
+                    const string defaultConfig = @"<?xml version=""1.0"" encoding=""utf-8""?>
+<configuration>
+  <packageSources>
+    <add key=""nuget.org"" value=""https://api.nuget.org/v3/index.json"" protocolVersion=""3"" />
+  </packageSources>
+</configuration>
+";
+                    System.IO.File.WriteAllText(nuGetConfigPath, defaultConfig, Encoding.UTF8); // UTF-8 with BOM
+                }
+
+                AddFallbackFolderToNuGetConfig(nuGetConfigPath, name, path);
+            }
+        }
+
+        private static void AddPackageToFallbackFolder(string fallbackFolder,
+            string nupkgPath, string packageId, string packageVersion)
+        {
+            // dotnet CLI provides no command for this, but we can do it manually.
+            //
+            // - The expected structure is as follows:
+            //     fallback_folder/
+            //         <package.name>/<version>/
+            //             <package.name>.<version>.nupkg
+            //             <package.name>.<version>.nupkg.sha512
+            //             <package.name>.nuspec
+            //             ... extracted nupkg files (check code for excluded files) ...
+            //
+            // - <package.name> and <version> must be in lower case.
+            // - The sha512 of the nupkg is base64 encoded.
+            // - We can get the nuspec from the nupkg which is a Zip file.
+
+            string packageIdLower = packageId.ToLower();
+            string packageVersionLower = packageVersion.ToLower();
+
+            string destDir = Path.Combine(fallbackFolder, packageIdLower, packageVersionLower);
+            string nupkgDestPath = Path.Combine(destDir, $"{packageIdLower}.{packageVersionLower}.nupkg");
+            string nupkgSha512DestPath = Path.Combine(destDir, $"{packageIdLower}.{packageVersionLower}.nupkg.sha512");
+
+            if (File.Exists(nupkgDestPath) && File.Exists(nupkgSha512DestPath))
+                return; // Already added (for speed we don't check if every file is properly extracted)
+
+            Directory.CreateDirectory(destDir);
+
+            // Generate .nupkg.sha512 file
+
+            using (var alg = SHA512.Create())
+            {
+                alg.ComputeHash(File.ReadAllBytes(nupkgPath));
+                string base64Hash = Convert.ToBase64String(alg.Hash);
+                File.WriteAllText(nupkgSha512DestPath, base64Hash);
+            }
+
+            // Extract nupkg
+            ExtractNupkg(destDir, nupkgPath, packageId, packageVersion);
+
+            // Copy .nupkg
+            File.Copy(nupkgPath, nupkgDestPath);
+        }
+
+        private static readonly string[] NupkgExcludePaths =
+        {
+            "_rels/",
+            "package/",
+            "[Content_Types].xml"
+        };
+
+        private static void ExtractNupkg(string destDir, string nupkgPath, string packageId, string packageVersion)
+        {
+            // NOTE: Must use SimplifyGodotPath to make sure we don't extract files outside the destination directory.
+
+            using (var archive = ZipFile.OpenRead(nupkgPath))
+            {
+                // Extract .nuspec manually as it needs to be in lower case
+
+                var nuspecEntry = archive.GetEntry(packageId + ".nuspec");
+
+                if (nuspecEntry == null)
+                    throw new InvalidOperationException($"Failed to extract package {packageId}.{packageVersion}. Could not find the nuspec file.");
+
+                nuspecEntry.ExtractToFile(Path.Combine(destDir, nuspecEntry.Name.ToLower().SimplifyGodotPath()));
+
+                // Extract the other package files
+
+                foreach (var entry in archive.Entries)
+                {
+                    // NOTE: SimplifyGodotPath() removes trailing slash and backslash,
+                    // so we can't use the result to check if the entry is a directory.
+
+                    string entryFullName = entry.FullName.Replace('\\', '/');
+
+                    // Check if the file must be ignored
+                    if ( // Excluded files.
+                        NupkgExcludePaths.Any(e => entryFullName.StartsWith(e, StringComparison.OrdinalIgnoreCase)) ||
+                        // Nupkg hash files and nupkg metadata files on all directory.
+                        entryFullName.EndsWith(".nupkg.sha512", StringComparison.OrdinalIgnoreCase) ||
+                        entryFullName.EndsWith(".nupkg.metadata", StringComparison.OrdinalIgnoreCase) ||
+                        // Nuspec at root level. We already extracted it previously but in lower case.
+                        entryFullName.IndexOf('/') == -1 && entryFullName.EndsWith(".nuspec"))
+                    {
+                        continue;
+                    }
+
+                    string entryFullNameSimplified = entryFullName.SimplifyGodotPath();
+                    string destFilePath = Path.Combine(destDir, entryFullNameSimplified);
+                    bool isDir = entryFullName.EndsWith("/");
+
+                    if (isDir)
+                    {
+                        Directory.CreateDirectory(destFilePath);
+                    }
+                    else
+                    {
+                        Directory.CreateDirectory(Path.GetDirectoryName(destFilePath));
+                        entry.ExtractToFile(destFilePath, overwrite: true);
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Copies and extracts all the Godot bundled packages to the Godot NuGet fallback folder.
+        /// Does nothing if the packages were already copied.
+        /// </summary>
+        public static void AddBundledPackagesToFallbackFolder(string fallbackFolder)
+        {
+            GD.Print("Copying Godot Offline Packages...");
+
+            string nupkgsLocation = Path.Combine(GodotSharpDirs.DataEditorToolsDir, "nupkgs");
+
+            void AddPackage(string packageId, string packageVersion)
+            {
+                string nupkgPath = Path.Combine(nupkgsLocation, $"{packageId}.{packageVersion}.nupkg");
+                AddPackageToFallbackFolder(fallbackFolder, nupkgPath, packageId, packageVersion);
+            }
+
+            foreach (var (packageId, packageVersion) in PackagesToAdd)
+                AddPackage(packageId, packageVersion);
+        }
+
+        private static readonly (string packageId, string packageVersion)[] PackagesToAdd =
+        {
+            ("Godot.NET.Sdk", GeneratedGodotNupkgsVersions.GodotNETSdk)
+        };
+    }
+}
diff --git a/modules/mono/editor/GodotTools/GodotTools/GodotSharpEditor.cs b/modules/mono/editor/GodotTools/GodotTools/GodotSharpEditor.cs
index f5c99e87ef..58561c5097 100644
--- a/modules/mono/editor/GodotTools/GodotTools/GodotSharpEditor.cs
+++ b/modules/mono/editor/GodotTools/GodotTools/GodotSharpEditor.cs
@@ -147,6 +147,21 @@ namespace GodotTools
                 case MenuOptions.AboutCSharp:
                     _ShowAboutDialog();
                     break;
+                case MenuOptions.SetupGodotNugetFallbackFolder:
+                {
+                    try
+                    {
+                        string fallbackFolder = NuGetUtils.GodotFallbackFolderPath;
+                        NuGetUtils.AddFallbackFolderToUserNuGetConfigs(NuGetUtils.GodotFallbackFolderName, fallbackFolder);
+                        NuGetUtils.AddBundledPackagesToFallbackFolder(fallbackFolder);
+                    }
+                    catch (Exception e)
+                    {
+                        ShowErrorDialog("Failed to setup Godot NuGet Offline Packages: " + e.Message);
+                    }
+
+                    break;
+                }
                 default:
                     throw new ArgumentOutOfRangeException(nameof(id), id, "Invalid menu option");
             }
@@ -183,6 +198,7 @@ namespace GodotTools
         {
             CreateSln,
             AboutCSharp,
+            SetupGodotNugetFallbackFolder,
         }
 
         public void ShowErrorDialog(string message, string title = "Error")
@@ -426,6 +442,7 @@ namespace GodotTools
             // TODO: Remove or edit this info dialog once Mono support is no longer in alpha
             {
                 menuPopup.AddItem("About C# support".TTR(), (int)MenuOptions.AboutCSharp);
+                menuPopup.AddItem("Setup Godot NuGet Offline Packages".TTR(), (int)MenuOptions.SetupGodotNugetFallbackFolder);
                 aboutDialog = new AcceptDialog();
                 editorBaseControl.AddChild(aboutDialog);
                 aboutDialog.Title = "Important: C# support is not feature-complete";
@@ -535,6 +552,16 @@ namespace GodotTools
             exportPlugin.RegisterExportSettings();
             exportPluginWeak = WeakRef(exportPlugin);
 
+            try
+            {
+                // At startup we make sure NuGet.Config files have our Godot NuGet fallback folder included
+                NuGetUtils.AddFallbackFolderToUserNuGetConfigs(NuGetUtils.GodotFallbackFolderName, NuGetUtils.GodotFallbackFolderPath);
+            }
+            catch (Exception e)
+            {
+                GD.PushError("Failed to add Godot NuGet Offline Packages to NuGet.Config: " + e.Message);
+            }
+
             BuildManager.Initialize();
             RiderPathManager.Initialize();