The Qt port of Scintilla is also used by a number of projects as a lightweight editor component:

  $ lsb_release -d && apt rdepends libqscintilla2-qt5-15
  Description:    Ubuntu 22.04.1 LTS
  libqscintilla2-qt5-15
  Reverse Depends:
    Depends: libqscintilla2-qt5-dev (<< 2.11.6+dfsg+1~)
    Depends: sqlitebrowser (>= 2.11.2)
    Depends: sonic-pi (>= 2.11.2)
    Depends: qgis-providers (>= 2.11.2)
    Depends: python3-qgis (>= 2.11.2)
    Depends: python3-pyqt5.qsci (>= 2.11.2)
    Depends: openscad (>= 2.11.2)
    Depends: octave (>= 2.11.2)
    Suggests: libqscintilla2-qt5-l10n
    Depends: libqscintilla2-qt5-dev (>= 2.11.6+dfsg)
    Depends: juffed (>= 2.11.2)
    Depends: libqscintilla2-qt5-designer (>= 2.11.2)
    Depends: libqgis-gui3.22.4 (>= 2.11.2)
    Depends: libqgis-app3.22.4 (>= 2.11.2)

It feels like an eternity ago, but an editor I wrote in Qt for a game did exactly this so you could write Lua inline on objects.

Doesn't Qt already have an editor component?

It does, but IIRC Scintilla is more full-featured (e.g. I think it has better syntax highlighting support).

Whenever I work on Windows machines, Notepad++ is a must to have.

But usually, SciTE is more than sufficient: https://www.scintilla.org/SciTE.html

and also works fine on Linux.

I remember using SciTE in the early 00s. Apart from some oddities of displaying italic text in a weird way it was a perfectly cromulent editor and you could start it, do your task, and close it before eclipse had even finished the loading screen :P

and Notepad++ itself sometimes gets bundled with other software.