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Is anyone aware of an editor in Linux that has this behaviour?

Geany. Not sure if by default or of it needs to be turned on in the settings. And Geany can be anything from a simple text editor to a very nice IDE.

Modern gedit on gnome has it

Which version? Or what enables it?

Version 41 (the one bundled w/Ubuntu 22.04 LTS) does have this behaviour from Notepad++