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diff --git a/web/src/deviceDetect.js b/web/src/deviceDetect.js
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+++ b/web/src/deviceDetect.js
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+// Coarse touch/mobile detection for the "match/rank against desktop only"
+// preference - not trying to be a precise device fingerprint, just enough
+// to separate "typed on a physical keyboard" from "typed on a touchscreen"
+// for fairness purposes (see anticheat.rs's comment on why this is opt-in
+// tagging, not a default exclusion).
+export function detectDeviceType() {
+    const coarsePointer = typeof window.matchMedia === 'function' && window.matchMedia('(pointer: coarse)').matches;
+    const touchPoints = navigator.maxTouchPoints > 0;
+    return coarsePointer && touchPoints ? 'mobile' : 'desktop';
+}