Hi all!
After a week of experimenting with Reaction coding (a domain I had never explored before) I managed to put together something, thanks to Dan (Trixie) and suggestions on forum from broadblues and Salas00.
The result might be of interest to some of you:
DumbPad v0.1:
https://kas1e.mikendezign.com/aos4/dumbpad/v01/dumbpad_v01.lhaSo, this is a simple text editor (yes, another one, I know). My goal was to create a truly minimal yet stable editor, or at least as stable as possible (hopefully, someday).
When I started, I envisioned something basic: just a window with two buttons for Open/Save and nothing more. I began with button.class buttons placed in a layout alongside a texteditor.gadget,
utilizing AISS for images, and it started to look promising:

However, I quickly noticed the absence of a scrollbar, of course. Since I had no experience with Reaction coding, I at first took an old-school way: like i do when coding diskmags 20 years ago: calculating everything
manually to size the scrollbar correctly. It worked, albeit with some precision issues. Then I realized that an RMB menu could be useful, even for a simple editor, to at least include Open, Save, and Quit options.
Added this using the menustrip.class with AISS support, and while I was at it, I added an About windwow (of course! most importan part! :) ) as well (using requester.class).

At this point, things were looking better, and I started using the editor myself. Soon, I found myself missing Cut/Copy/Paste functionality (mostly Cut, to be honest) , but I ended up adding all three. Additionally,
I was annoyed that the text editor area wasn't active by default when clicking the window's border, resizing, or clicking the toolbar area. To address this, I added logic to check for an active window,
ensuring the text editor area is always enabled when expected (thanks for hint, Andy).

Seeing the progress, I realized I also missed a "New" button. I added that too, but launching another instance of the editor just like os4's default notepad felt too much, which led me to consider tabs.
Dan (Trixie) was incredibly helpful again, explaining the key details I needed, and I managed to piece together something functional, complete with bubble help:

At this stage, I encountered bugs with the tabs, various issues, and a codebase that was turning into a spaghetti mess. I also realized that much of my work was redundant! I didn’t need to code scrollbar
support myself—BOOPSI handles that with just a few lines of code. I didn’t need to manually manage every state change in WMHI_* events, as Notifications cover most cases. And I certainly didn’t need to
write a lot of code that essentially did nothing, since BOOPSI takes care of most things. I simply didn’t fully understand at start that there no Intuition-based coding which requires micromanaging every
detail: BOOPSI makes things much easier. Of course, some classes lack certain features one might want, others are overly complicated to set up, but it’s still a far cry from the old-school Intuition coding days,
which is a good thing!
So, with all this, I decided to start from scratch while it wasn’t too late. I opted for a simple window with speedbar.gadget support (since it redraws all buttons in a single call, unlike the button-layout
approach where each button requires a separate call leading to flickering when you have more than a few buttons). I then added tabs, a menu, little bit of functionality, Notifications (thanks to Andy and Dan!), tab naming,
and handling of edge cases like loading empty files or any file type (including binary) by skipping 0x00 bytes (since the text editor interprets them as string terminators). I also implemented iconification,
removed dependency on an external image for the tab close gadget (inbuid them inside of binary), added AISS support for menus and buttons, and made various small tweaks. The result: v0.1 of the dumbest editor ever:
(click for full size):

It definitely has bugs for now, and there are features I haven’t yet implemented (but plan to):
Undo/Redo functionality.
Display file names not only in tabs but also in the window title.
Support for a single command-line argument to specify the name of a file to create.
A bug shared with Notepad (likely an issue with texteditor.gadget or possibly flawed code on my part and the Notepad author’s): if you type quickly and repeatedly click the Save button, after 5–10 attempts -
the Save button becomes ghosted when it shouldn’t. I suspect this stems from how both Notepad (I haven’t seen its source) and my editor handle GA_TEXTEDITOR_HasChanged notifications via idcmphook,
toggling the button state based on editor changes. Alternatively, it might be a logic error in flag management (odd that it affects both editors).
At the very least, I aim to address these four points (plus some minor tweaks), then clean up the code and make it open-source. This way, if any of you find it useful, bugs can be identified and fixed more easily.
Thanks for reading and testing !