How React's Concurrent Scheduler Causes Suspense Flickers and How to Debug Them

React's concurrent rendering mode breaks component rendering into small, interruptible units rather than processing the entire tree at once, allowing high-priority tasks like user input to jump ahead in the queue. When a component suspends by throwing a Promise, React shows the Suspense fallback while continuing to render the suspended component in the background. This pause-and-resume cycle can be interrupted multiple times by higher-priority updates, causing fallback UIs to flash or flicker repeatedly on screen. A notable side effect is that interrupted Suspense boundaries may cause React to discard in-progress work and remount components, potentially resetting local state such as form inputs. Developers can diagnose these issues using the React DevTools Profiler and React 18's tracing APIs to track when rendering suspends, resumes, and commits.