Once Shiki and a language's grammar have loaded, a code block in that language is colored from its first frame, rather than drawn plain and laid out again in color a moment later, as every block was when a reply's record replaced its stream or a chat opened. The loaded highlighter is kept, so a block highlights while it first renders and in its layout effect, before the browser paints, and only the first block of a session waits for the import. The web UI spec says so, and a test checks a block is colored on its first render once its language has loaded.