A text editor that supports doodling. Some previous ways to have text coexist with a canvas: • A fixed background that is disconnected from text. • Fixed page boundaries. Here I'm instead attaching doodles to a pivot character that they move with in the presence of edits. When you start drawing you're presented with a set of pivot options, one grey square per character on screen. After you pick one, it turns pink and the other options disappear. Now you can start drawing. Displacements to the pivot are preserved in font-derived units, so it looks "reasonable" as you resize the font. Deleting a character deletes all drawings pivoted on it. (But there's undo.) This took 200 lines, so not too much though it was more than I'd initially expected. Inspired by this thread.

Neat. Less radically, have you considered sketches in the margins, linked to characters? Or sketches that appear in boxes, with text wrapping around them? I ask cuz “sketch directly on top of text” doesn’t seem as useful as those. Aside from, say, editing marks. Maybe increasing line spacing would make sketches literally anchored to characters more useful?

It's a good question. This particular configuration came up in conversation, I think, as something that looks and feels like paper. I'm not sure either that it's useful as is.

As a heavy user of pencil-on-tablet annotations of scientific papers, my vote is for annotations attached to a place in the text but shown/drawn on the margins. Plus manual text markup: highlight/underline, strike through.