Flip Stopwatch

The stopwatch splits its display in two. Minutes and seconds flip on the split-flap cards; hundredths tick beside them as plain fast-updating text. That split is deliberate: flipping every hundredth would be unreadable motion, while the main digits keep the calm one-flip-per-second rhythm of the clock.

Laps record both the split and the running total. The table marks the fastest split green and the slowest red, so intervals stand out at a glance during training. Copy puts a plain-text table on the clipboard; CSV downloads a file with lap number, split, and total columns that opens directly in spreadsheets.

Elapsed time is computed from timestamps with performance.now precision, never by incrementing a counter. Background tabs, screen locks, and full reloads cannot lose time — the running state, accumulated milliseconds, and every lap persist locally and resume on return. The display loop runs on requestAnimationFrame while the tab is visible and settles to a static reading when hidden.

Three keys run the whole tool: Space starts and stops, L takes a lap, R resets. F takes it fullscreen for tracks, pools, and labs, where the control bar hides after three idle seconds and returns on any movement. Like everything here, there is no account and no server — laps never leave this browser.