Thanks to a lot of help by Robert Futrell, Fiji’s Script Editor learnt ImageJ’s macro language. Now you can enjoy the power of full undo and of syntax highlighting when writing macros!
For further convenience, the Macro Recorder opens newly created macros in the Script Editor, too.
Another nice consequence is that ⌃ Ctrl / ⌘ ⌃ Ctrl on a PC ⌘ command on a Mac + ⇧ Shift + N opens up the Script Editor.