Adding on yet again to what @SummerEquinox said, you can build your custom implementation off of the one implemented in the core gui. The files are somewhere in [user]/AppData/local/Roblox/versions/[version]/content/scripts. But it will eventually fall out of date and require maintenance to add new features. I would use Git to add your changes onto the file, and merge your version with Roblox’s updated one every so often.