How to let XSLfast manage visual parts, and let me write the logic behind in notepad++?

Often, we have questions like this:

I want to change XSL file in Notepad++, but let XSLfast manage it too. How to do this?

In general, XSLfast does not able to open any XSL file and open it as a layout - it only imports templates as XSL code.

Should it mean the answer is "no, it doesn't"?

Way is there, but it is based upon separation of XSL pieces: visual part and logic behind.

So, for this case your flow might be as follows:

Move all data you need to change into templates, visually.
Export XSL file from XSLfast.
Consume templates you need in text editor, and move it into different file foo.tpl.
Return back to XSLfast, and remove all visual elements you moved to external templates.
In Layout settings, add your foo.tpl file as external templates file, and save the prepared layout.

Now, the visual elements are in XSLfast, and the logic in separate file. And you can use your foo.tpl freely in text or XML editor as you need, and XSLfast still able to manage its' layouts.

This hybrid way is alone right now.