Map where input reaches the browser
The canvas contains 96 regions arranged in a 12 by 8 grid. Slowly drag one fingertip across each row and column. The counter reports covered regions and input events. Reset before repeating a suspect region so previous coverage does not hide an intermittent problem.
The grid covers the browser overlay, not areas behind browser chrome or its control buttons. Mouse input is available for demonstration on desktop and does not establish touchscreen health. Use one finger here: joining separate contacts can create a misleading apparent path.
An empty region is a place to retest
Check that your finger actually passed over the region. Dry the screen and your hands, disconnect the charger and repeat gently; do not press harder. Rotate the phone and compare the same physical patch of glass. If it repeatedly fails in other apps too, a digitizer or connection issue becomes more plausible.
Coverage marks only positions delivered by the browser. It cannot measure exact touch-to-display latency, calibrate the digitizer or prove every pixel responds. Fast movement may leave unobserved gaps between event samples, so slow passes are more useful.
Separate dead areas from ghost touches
A region that lights up when you did not touch it is unexpected input, while one that remains empty may be untested or unresponsive. The ghost-touch guide helps compare moisture, charging and screen-protector effects. For several fingers dropping out together, run the simultaneous touch test. If a dark patch still responds to taps, inspect the display with the dead-pixel tool.
Your browser may reserve edge gestures or cancel contacts. A saved result includes coverage and your assessment, not an automated physical diagnosis. Coordinates are processed locally.