Design the Canvas Rendering Engine

This is the rare frontend system-design round, and treating it as a client-performance problem from the first sentence is what separates a strong answer from a generic one. There are no shards here, no replicas, no consistency model to defend. The document lives in memory on the user's machine, and the only clock that matters is the frame budget. At 60fps you have about 16 milliseconds to turn the current document plus the current viewport into pixels, every single frame, or the pan visibly stutters. Figma is one of the very few companies that runs this round at all, because it actually built a custom rendering engine in the browser to make a professional design tool feel native. If your loop is frontend or core-engine, the interviewer wants to hear you reason about what to draw, what to skip, and what to reuse, all under that 16-millisecond ceiling. Open by naming the budget and the scale, and let every later choice visibly serve them.