Let's face it, every decent programmer knows how to modularize his/her code. Even with C being as primitive as it is, you can still split your code in several C & H files. In ThinkerCAD circuits we usually have multiple modules or a variety of functionality that is needed in order to accomplish the task. AT present, ALL the code has to be included in a single page, usually requiring a lot of scrolling. Why not allow the user to split the CODE in several modules. I could think of several tabs (given there wouldn't be too many source files to a single project) but that would probably take too much screen real estate.
I propose the ability to split the CODE in several modules and then at the TOP of the CODE panel, place a ComboBox control in which the user can select -via the drop down- the source file he/she wants to edit.