As of v1.9.3, Unity has added a simple extention API to the PackageManager UI. This small example shows you how to implment it and add a new window to the package manager. Note: you will need to change to the Staging package repository as time of writing. This code needs to go in an Editor folder

PackageManagerExtentionExample.cs

using System.Linq;
using UnityEditor;
using UnityEditor.PackageManager.UI;
using UnityEngine;
using UnityEngine.Experimental.UIElements;
using UnityEngine.Experimental.UIElements.StyleEnums;
using PackageInfo = UnityEditor.PackageManager.PackageInfo;

[InitializeOnLoad]
public class PackageManagerExtentionExample : IPackageManagerExtension
{
    // This class implments the new IPackageManagerExtension interface, allowing us to plug it
    // into the package manager window. this feature is still very much under development
    
    static PackageManagerExtentionExample()
    {
        Debug.Log("CALLED STATIC CTOR"); 
        
        // This static constructor gets called becuase we have the InitializeOnLoad attribute
        // on our class. Here we register this PackageManager extention so our exta version list
        // shows up in the manager window
        PackageManagerExtensions.RegisterExtension(new PackageManagerExtentionExample());
    }
    
    // this is the scrollview were going to put the version list into
    ScrollView tasksContainer;

    public VisualElement CreateExtensionUI()
    {
        Debug.Log("CALLED CreateExtensionUI");
        
        VisualContainer root = new VisualContainer()
        {
            style =
            {
                backgroundColor = Color.grey,
                alignSelf = Align.FlexStart,
                flexDirection = FlexDirection.Row 
            }
        };
        
        tasksContainer = new ScrollView()
        {
            style =
            {
                width = 400,
                height = 100,
                flexDirection = FlexDirection.Column,
            }
        };
        tasksContainer.showHorizontal = false;
        root.Add(tasksContainer);

        return root;
    }

    public void OnPackageSelectionChange(PackageInfo packageInfo)
    {
        // setup the version list for the currently selected package
        Debug.Log("CALLED OnPackageSelectionChange");
        
        tasksContainer.contentContainer.Clear();

        foreach (var version in packageInfo.versions.all.Reverse())
        {
            tasksContainer.contentContainer.Add(NewVersion(version, packageInfo));
        }
    }

    private VisualElement NewVersion(string version, PackageInfo packageInfo)
    {
        // create a list item for the passed in version
        var versionElement = new VisualContainer()
        {
            style =
            {
                width = 400,
                flexDirection = FlexDirection.Row
            }
        };
        versionElement.name = version;

        // fist, lets put in the version name
        versionElement.Add(new Label(version));
        
        // annnd now a bunch of interesting info
        versionElement.Add(new Toggle(null){text = "Recomended", on = (packageInfo.versions.recommended == version)});
        versionElement.Add(new Toggle(null){text = "Compatible", on = (packageInfo.versions.compatible.Contains(version))});
        versionElement.Add(new Toggle(null){text = "Latest Compatible", on = (packageInfo.versions.latestCompatible == version)});
        
        Debug.Log("ADDED " + version);
        
        return versionElement;
    }

    public void OnPackageAddedOrUpdated(PackageInfo packageInfo)
    {
        Debug.Log("CALLED OnPackageAddedOrUpdated");
    }

    public void OnPackageRemoved(PackageInfo packageInfo)
    {
        Debug.Log("CALLED OnPackageRemoved");
    }
}

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Always thankful for your support, Lotte💖

Lotte - your gist page is a goldmine! Thanks so much for doing this!! Coffee bought!