Pick a letter at random.
I wasn't checking that the destination values were not less than zero,
which let you click at the top/left edges of the screen and therefore
the carefree_alpha_blend_blit() would try to write to areas outside the
framebuffer. I started to see "zalgol" pixels in the letters, so I'm
guessing the pixel data is getting stored just above the framebuffer,
which makes sense (because that's the order they appear in the source
code and this is a simple system!) When you click at the top of the
screen it was writing pixels in the font data, eh? Then when you click
elsewhere on the screen you get extra pixels with your letterforms and
it looks like Unicode Lovecraft puns.
This is a compromise between updating the screen every frame (which
takes ~60% CPU on my old no-GPU hardware) and repairing damage from e.g.
dragging offscreen and back on, or covering and uncovering the window
with another window.
Simon Forman