diff --git a/src/background/style-via-webrequest.js b/src/background/style-via-webrequest.js
index 97f1b7aec8..7fd2f212cd 100644
--- a/src/background/style-via-webrequest.js
+++ b/src/background/style-via-webrequest.js
@@ -102,7 +102,7 @@ function toggle(prefKey) {
     return;
   }
   let v;
-  if (__.BUILD !== 'chrome' && FIREFOX || (
+  if (__.BUILD !== 'chrome' && FIREFOX || off !== curOFF && (
     __.MV3
       ? csp !== curCSP
       : (xhr || csp) !== (curXHR || curCSP)
@@ -110,14 +110,16 @@ function toggle(prefKey) {
     v = chrome.webRequest.onHeadersReceived;
     // unregister first since new registrations are additive internally
     toggleListener(v, false, modifyHeaders);
-    toggleListener(v, true, modifyHeaders, WR_FILTER, [
+    toggleListener(v, !off, modifyHeaders, WR_FILTER, [
       'blocking',
       'responseHeaders',
       !__.MV3 && xhr && chrome.webRequest.OnHeadersReceivedOptions.EXTRA_HEADERS,
     ].filter(Boolean));
   }
-  if (mv3init || off !== curOFF) {
-    toggleListener(chrome.webRequest.onBeforeRequest, mv3init || !off, prepareStyles, WR_FILTER);
+  if (mv3init || off !== curOFF && (xhr || csp) !== (curXHR || curCSP)) {
+    toggleListener(chrome.webRequest.onBeforeRequest,
+      mv3init || !off && (xhr || csp),
+      prepareStyles, WR_FILTER);
   }
   curCSP = csp;
   curOFF = off;
@@ -126,6 +128,7 @@ function toggle(prefKey) {
 
 /** @type {typeof chrome.webRequest.onBeforeRequest.callback} */
 async function prepareStyles(req) {
+  if (!curXHR && !curCSP && !bgBusy) return;
   const {tabId, frameId, url} = req; if (tabId < 0) return;
   const key = tabId + ':' + frameId;
   const bgPreInitLen = __.MV3 && bgPreInit.length;
diff --git a/src/js/msg.js b/src/js/msg.js
index b28f03ae65..ace7f780c9 100644
--- a/src/js/msg.js
+++ b/src/js/msg.js
@@ -16,14 +16,17 @@ const handler = {
 // TODO: maybe move into browser.js and hook addListener to wrap/unwrap automatically
 chrome.runtime.onMessage.addListener(onRuntimeMessage);
 
+/** @param {function} fn - return `undefined` by default to avoid breaking onRuntimeMessage */
 export function onMessage(fn) {
   handler.both.add(fn);
 }
 
+/** @param {function} fn - return `undefined` by default to avoid breaking onRuntimeMessage */
 export function onTab(fn) {
   handler.tab.add(fn);
 }
 
+/** @param {function} fn - return `undefined` by default to avoid breaking onRuntimeMessage */
 export function onExtension(fn) {
   handler.extension.add(fn);
 }
diff --git a/src/manage/index.js b/src/manage/index.js
index 3092979d03..2ff0f15680 100644
--- a/src/manage/index.js
+++ b/src/manage/index.js
@@ -26,7 +26,9 @@ tBody();
   if (newUI.hasFavs()) readBadFavs(badFavs);
   showStyles(styles, ids);
   renderSyncStatus(sync);
-  onExtension(e => e.method === 'syncStatusUpdate' && renderSyncStatus(e.status));
+  onExtension(e => { // returning `undefined` by default to avoid breaking bg::onRuntimeMessage
+    if (e.method === 'syncStatusUpdate') renderSyncStatus(e.status);
+  });
   import('./lazy-init');
 })();