A browser may reduce or stop visual callbacks for a hidden tab. A stopwatch that adds a fixed amount on every callback will then fall behind. LapWatch instead stores the monotonic start point and accumulated paused duration, then subtracts timestamps whenever it needs a display or lap.
Rendering is not the clock
Animation frames only decide when the digits are repainted. They never increment the authoritative duration. When the tab becomes visible again, the next timestamp may jump forward and the display catches up in one render. This avoids callback-count drift, although it cannot prevent a browser or operating system from suspending the whole execution environment.
Pause accumulates once
Pressing Pause adds the current run interval to the accumulated duration and removes the active start point. Resume creates a new start point without changing the stored duration. This explicit state transition prevents repeated pause clicks or repaint callbacks from adding the same interval twice.
Know what the browser cannot promise
LapWatch is an informal in-page instrument, not an independent clock service. Closing or reloading the page clears every timestamp and lap. Device sleep, browser suspension and operating-system differences can affect what one browser reports. Keep another suitable method whenever the duration matters, and never use this tool for medical, emergency, food-safety, transport, industrial, official, legal or other safety-critical timing. The fixed 99:59:59.999 and 500-lap limits prevent a forgotten tab from pretending to provide unlimited continuity.