Highlight Wave

a page of notes you can mark up. select a run of text and a palette comes up over it, and picking a colour sends a wave along the selection: each character swells, glows in that colour and settles into it while the bar fills in behind.

key insight: the bars are drawn from the mark's own line fragments in a layer under the text rather than as a background on it, so each line rounds its own ends and the bar can sweep in a line at a time behind the characters. every character holds its old colour until the wave reaches it, so recolouring a mark runs the same wave from one colour to the next.

kerning is off on the notes, so a run split into characters for the wave sets at exactly the width it had as plain text and nothing reflows when it is marked.