It turns out the 2023.8 update broke the legacy system's level editor and nobody noticed until today. I've uploaded a fixed version of the legacy source code.

This is a small bugfix that only makes two changes; if you're for some reason using a project made in the legacy version in the latest version of Game Maker, you can easily apply the fix in your project like this.

In editoroverlay's Create event, remove the minus signs preceding any button event that's set to a script. (Negating a reference is not allowed from 2023.8 onward and this is the bug that's breaking the editor)

To actually get the right behavior after this change, change the check at the start of the Global Left Pressed event to !is_real instead of < 0. (I also updated the comments here but that's optional)

...and that's it.