• no need to bother with Scintilla’s native notifications and their nuances;
• full control and clarity of what happens and when it happens: within own WM_CHAR handler, we are able to process a character before it goes to Scintilla and after it goes to Scintilla;
• simplified code because all the logic is controlled by you and there is no need to conform to Scintilla’s way of processing the events.

being typed or the result of pasting

hmm … I think you need to subclass the Scintilla control and check for the WM_PASTE message. The scintilla api, to my knowledge, has no way of letting you know that. But is it even important to know the difference?

Thank you, SC_MOD_INSERTCHECK is an interesting way indeed.
In this case, how can I understand whether SC_MOD_INSERTCHECK is the result of a character being typed or the result of pasting from the clipboard?

I assume you could also use SCI_CHANGEINSERTION within the modified notification.
To do this, however, you must request the necessary additional SC_MOD_INSERTCHECK in nppn_ready.

hint

Well, there is a “nuclear option”…

Nothing stops a plugin from using SetWindowSubclass to subclass the main Notepad++ window in order to intercept and preprocess messages.

Example:

• set subclass
• subclass procedure
• what I’m actually doing (making sure tabstops that might be off screen are set before a rectangular selection paste)

I say “nuclear option” because you really need to be careful that you’re not messing up expectations for some other function or plugin. This should be a last resort, because it leaves you free to mess things up entirely.

Buffer* currentBuf = _pEditView->getCurrentBuffer();
if (currentBuf->allowAutoCompletion())
{
	AutoCompletion* autoC = isFromPrimary ? &_autoCompleteMain : &_autoCompleteSub;
	bool isColumnMode = _pEditView->execute(SCI_GETSELECTIONS) > 1; // Multi-Selection || Column mode)
	if (nppGui._matchedPairConf.hasAnyPairsPair() && !isColumnMode)
		autoC->insertMatchedChars(notification->ch, nppGui._matchedPairConf);
	autoC->update(notification->ch);
}

And what is even more interesting, the _pluginsManager.notify(notification); sends a copy of the notification to plugins. Which means there’s no sense to modify e.g. notification->nmhdr.code inside a plugin.
So… Seems I have two options:

1. temporarily tune some settings to make currentBuf->allowAutoCompletion() return false;
2. find a way to hide the AutoCompletion Popup if it was shown.

Some hint or additional advice of how to achieve that would be appreciated.

Could you suggest why the Auto-Completion Popup is shown and what can I do to prevent it to be shown? Or, at least, to detect that it is shown and then to hide it immediately?

I don’t have an answer to your question, but maybe a place to get started.

Notepad++ processes Scintilla notifications here; it sends notifications to the plugins first, then sends them to its own processing routines. The routine for SCN_CHARADDED is here. The plugin notification routines (here calling here) copy the notification to make sure the plugin can’t change it.

At least that should give you a clue where to look to follow what Notepad++ is doing and see if you can find a way to suppress the unwanted behavior.