I was having a problem with the "test game" feature.
In all tested 2.6 and 2.7 versions, when clicking on the "test game" command in the file menu for the first time, the mouse cursor disappeared and the screen froze.
Opening the task manager and terminating the AGS process returned control to the OS, but opening AGSedit again and attempting to use the "test game" command would produce an identical freezeup.
When the screen was frozen, if you pressed the windows button on the keyboard, it opened the start menu and overlayed a working taskbar over the frozen one (winxp). Moving the mouse down until it reached the taskbar area made it visible (only in that area). Right-clicking on the taskbar button for the test game, allowed you to close it without closing AGS. After doing that, the "test game" feature works flawlessly, no matter how many times you attempt it. If you close AGS and open it again later (even weeks later), the "test game" feature still works correctly. If the computer is rebooted, the freezup problem returns.
Even after two reboots, the experimental version of the editor provided in this thread solved the problem.