The Source panel shows the VFJ source code of the current glyph and all its layers. While editing the entire VFJ in a text editor is handy, it may also be a bit intimidating because there is no visual link between the code and the graphical representation.
The Source panel gives you that visual link. It provides a fast, immediate access to the VFJ source of the current glyph: nodes and handles coordinates, element transformations etc.
The panel can show the code of the current glyph, current layer or current element.
To select whether you want to see the source code of the current glyph, layer or element, use one of the buttons at the top-left of the panel.
When you edit a glyph in the Glyph Window, the Source panel updates to reflect your changes. When you turn on Preferences > Editing > Live preview, the update will be instant and immediate as you move your mouse pointer; when you turn the preference off, the panel will update once you release the mouse pointer.
When you edit source text in the Source panel, the Apply button will be enabled, which means that the data in the panel is more recent than what you see in the Glyph window. To apply the changes you have made to the code, click on the Apply button. The Source panel will notify you if you’ve entered some code that is invalid.
To revert to back your changes before you’ve applied them, click on the Revert button.
The Source panel currently does not give you access to data that is not “inside” any particular glyph, for example classes, kerning or Font Info. To edit that data in text form, save or export the font as VFJ and use a text editor.