diff --git a/docs/architecture.adoc b/docs/architecture.adoc
index d636e3d..f591418 100644
--- a/docs/architecture.adoc
+++ b/docs/architecture.adoc
@@ -261,6 +261,21 @@ as "the GNSS simulator, leaking somehow"). Uses MapLibre's own blue
 pulsing `.maplibregl-user-location-dot` (`maplibre-gl.css`) instead —
 the same look GeolocateControl itself had here before, and visually
 distinct from GNSS's green on purpose.
++
+GNSS turning on *while* the browser fallback is already mid-track
+switches over immediately rather than waiting for another click —
+`GnssControl#onEnabledChange` (a small internal callback list, not a
+full pub/sub system — nothing else needs one) notifies `LocateControl`
+the moment the sidebar toggle changes; if it was actively tracking via
+the browser at that point, it stops that and jumps to GNSS's position
+instead. That jump doesn't alert if GNSS has no fix yet (a
+`{ alertIfNoFix }` option on the shared jump-to-GNSS helper, `true`
+only for a deliberate click) — `GnssControl` only just turned on and
+almost certainly hasn't gotten its first poll response back yet at
+the exact moment this fires, so alerting here would pop a blocking
+dialog (window.alert() halts the whole page until dismissed) on every
+single enable; its own next poll shows/recenters once it actually has
+something, same as it would either way.
 
 === Debug inspector (Alt+click)
 
diff --git a/public/js/gnss.js b/public/js/gnss.js
index 186fd14..ce1aa65 100644
--- a/public/js/gnss.js
+++ b/public/js/gnss.js
@@ -15,6 +15,16 @@ export class GnssControl {
     this._marker = null;
     this._timer = null;
     this._enabled = false;
+    this._enabledListeners = [];
+  }
+
+  // Read by LocateControl: if it's mid-tracking on the browser-geolocation
+  // fallback when GNSS turns on, GNSS should take over there and then, not
+  // only the next time the button happens to get clicked — the browser's
+  // own (often less accurate) position has no business still being used
+  // once the app's own receiver has a fix.
+  onEnabledChange(callback) {
+    this._enabledListeners.push(callback);
   }
 
   mount(sidebar) {
@@ -46,6 +56,7 @@ export class GnssControl {
       this._removeMarker();
       this._status.textContent = "";
     }
+    this._enabledListeners.forEach((callback) => callback(enabled));
   }
 
   _stopPolling() {
diff --git a/public/js/locate.js b/public/js/locate.js
index 87fd68e..5927746 100644
--- a/public/js/locate.js
+++ b/public/js/locate.js
@@ -37,6 +37,25 @@ export class LocateControl {
     this._container = document.createElement("div");
     this._container.className = "maplibregl-ctrl maplibregl-ctrl-group";
     this._container.appendChild(this._button);
+
+    // GNSS turning on mid-track (this control's own browser-geolocation
+    // fallback, not GnssControl's — that one manages its own tracking)
+    // should switch over immediately, not just the next time this button
+    // gets clicked; the browser's own position has no business still
+    // being used once the app's own receiver has a fix. No alertIfNoFix
+    // here (unlike a deliberate click, below) — GnssControl only just
+    // turned on and almost certainly hasn't gotten its first fix back yet
+    // (its own poll is async, still in flight at the moment this fires),
+    // so this would otherwise pop a blocking "waiting for a fix" dialog on
+    // every single enable; GnssControl's own next poll will show/recenter
+    // once it actually has something, same as it would either way.
+    this._gnssControl.onEnabledChange((enabled) => {
+      if (enabled && this._active) {
+        this._stop();
+        this._jumpToGnss();
+      }
+    });
+
     return this._container;
   }
 
@@ -51,16 +70,7 @@ export class LocateControl {
 
   _click() {
     if (this._gnssControl.isEnabled()) {
-      // GNSS already follows the map on its own once enabled (GnssControl)
-      // and has no separate "off" state to toggle from here — just jump to
-      // its current fix now, for the same instant feedback clicking this
-      // button has always given.
-      const position = this._gnssControl.getPosition();
-      if (position) {
-        this._map.flyTo({ center: position, zoom: Math.max(this._map.getZoom(), 15) });
-      } else {
-        window.alert("Waiting for a GNSS fix…");
-      }
+      this._jumpToGnss({ alertIfNoFix: true });
       return;
     }
 
@@ -71,6 +81,23 @@ export class LocateControl {
     }
   }
 
+  // GNSS already follows the map on its own once enabled (GnssControl) and
+  // has no separate "off" state to toggle from here — just jump to its
+  // current fix, for the same instant feedback clicking this button has
+  // always given. alertIfNoFix is opt-in (a deliberate click wants to know
+  // why nothing happened; the automatic mid-track handoff above doesn't —
+  // see its own comment) since a JS alert() blocks the whole page until
+  // dismissed, not something to pop up on a path that isn't a direct user
+  // action.
+  _jumpToGnss({ alertIfNoFix = false } = {}) {
+    const position = this._gnssControl.getPosition();
+    if (position) {
+      this._map.flyTo({ center: position, zoom: Math.max(this._map.getZoom(), 15) });
+    } else if (alertIfNoFix) {
+      window.alert("Waiting for a GNSS fix…");
+    }
+  }
+
   _start() {
     this._active = true;
     this._button.classList.add("maplibregl-ctrl-geolocate-active");