Check every part of your touchscreen right in the browser: see how many fingers it tracks, sweep the glass to find dead zones, and catch the phantom touches a failing digitizer invents.
Run it on the device you are testing. Clean the glass first, and take off any case that overlaps the edges.
Two checks catch almost every touchscreen fault: how many fingers the screen follows, and whether it follows them everywhere. Both run in the browser, on the device you are testing, with nothing to install.
Put your fingers on the screen and each contact gets a circle, a number and a live readout of its position and contact size. Move them around and the trail behind each finger shows how smoothly the screen follows: a healthy panel draws a clean line, a failing one jumps, stutters or drops the contact halfway. Lift a finger and its circle should disappear immediately.
A digitizer rarely fails all at once. It loses a band, a corner or a strip along one edge, and everything else keeps working — which is why the fault is so easy to miss until you land on the wrong key. The grid test turns that into something you can see: sweep the whole screen and any tile that stays dark never reported your finger. Sweep slowly, and go over the edges twice, since that is where damage usually starts.
A dying digitizer starts reporting touches nobody made: the keyboard types by itself, apps open, a menu flicks past. The multi-touch view counts every contact that appears and disappears too fast to be a real tap, so you can leave the test open, not touch the screen, and see whether the number climbs on its own. A few counts while you are actively tapping mean nothing — it is the ones that arrive while your hands are off the glass that matter.
Almost every modern phone and tablet tracks at least five fingers, and many handle ten. Windows laptops with a touchscreen usually manage ten as well. The number the browser reports is shown under the start button, and the test shows how many the screen actually delivers at the same time — those two can differ when a digitizer is failing. Anything below five on a recent device is worth a closer look.
Start with the cheap causes. Wipe the glass and dry your hands — water and grease scatter the touch signal. Take off the screen protector, especially a thick or badly fitted one, and any case that presses on the edges. Restart the device, then check for a system update. If the dead area survives all of that, and it always sits in the same place, the digitizer or its ribbon connector is damaged and the panel needs replacing.
The trackpad under your keyboard is invisible to this test, and to every other web page. The operating system turns its gestures into a mouse cursor before the browser ever sees them, so there is nothing to measure: no coordinates, no contact count, no second finger. If your trackpad is misbehaving, the answer is in the system settings and the driver, not in a browser test.