Back when I first (and last) had to write my own text editor, 30 years ago, memory was at a premium, so I did what everybody else in the old Commodore days did:

Back when I first (and last) had to write my own text editor, 30 years ago, memory was at a premium, so I did what everybody else in the old Commodore days did: use the screen buffer, a paltry 40*25=1000 bytes, as additional memory. When you first loaded a file, it would go straight to the screen buffer, B. Well, almost straight: you had to move to a new line when a CR character was seen. The rest of the file would go into buffer C, the region at the end of the free memory space. As you scrolled down, the first line would go into buffer A, the region at the beginning of the heap, minus trailing whitespace. And so on. In hindsight, this is a variant of gap buffers. If you edit the first line in a file, changes stay within the current paragraph on screen, until the paragraph needs to take up an extra line. Even then, though, the line at the bottom that scrolls off the screen only needs to be prepended to C, which, again, is at the end of the memory map and thus grows backwards.