Dear all,
Thanks to the many helpful suggestions, especially based on Mr. Randy Kramer's, I made one work-around in Notepad++, which is also based on Scintilla.
The steps are as following:
1. Settings -> Preferences -> New Document/Default Directory -> Default language -> select "Python" ( I chose Python because it is the language that I don't often use, while it supports folding based on indentation)
2. Settings -> Style configurator -> Lauguage: select Python -> User ext.: input "txt"
(I found that making an adjustment like this is a bit more complicated in Geany, unlike Notepad++. Especially I couldn't find a way to select a default language. Could someone tell me how I can do that?)
Also, although this works perfect for me, this is only a work-around, and poses several problem. For example, if you want to use auto-completion for 'real' Python code, you will be forced to see the same auto-completion list pops up even when you are writing a plain text.
Since folding with indentation 'right from the start' will be a qualitatively different experience to novices to Geany, I still hope that this feature could be included as a default, or an option that can be selected from the GUI.
.txt lexer, combined with a capacity to select a default language (in this case, 'simple text (.txt)' ), will be an answer.
I will try Eugene's patch and post again.
Best regards, HR Kang.