support all API methods in content scripts

content/apply.js

@@ -134,17 +134,12 @@ self.INJECTED !== 1 && (() => {
   }
 
   function fetchParentDomain() {
-    if (parentDomain) {
+    return parentDomain ?
-      return Promise.resolve();
+      Promise.resolve() :
-    }
+      API.getTabUrlPrefix()
-    return msg.send({
+        .then(newDomain => {
-      method: 'invokeAPI',
+          parentDomain = newDomain;
-      name: 'getTabUrlPrefix',
+        });
-      args: []
-    })
-      .then(newDomain => {
-        parentDomain = newDomain;
-      });
   }
 
   function updateExposeIframes() {
@@ -168,14 +163,9 @@ self.INJECTED !== 1 && (() => {
     }
     if (STYLE_VIA_API) {
       API.styleViaAPI({method: 'updateCount'}).catch(msg.ignoreError);
-      return;
+    } else {
+      API.updateIconBadge(styleInjector.list.length).catch(console.error);
     }
-    // we have to send the tabId so we can't use `sendBg` that is used by `API`
-    msg.send({
-      method: 'invokeAPI',
-      name: 'updateIconBadge',
-      args: [styleInjector.list.length]
-    }).catch(console.error);
   }
 
   function orphanCheck() {

js/msg.js

@@ -238,9 +238,15 @@ self.msg = self.INJECTED === 1 ? self.msg : (() => {
   }
 })();
 
-self.API = self.INJECTED === 1 ? self.API : new Proxy({}, {
+self.API = self.INJECTED === 1 ? self.API : new Proxy({
+  // Handlers for these methods need sender.tab.id which is set by `send` as it uses messaging,
+  // unlike `sendBg` which invokes the background page directly in our own extension tabs
+  getTabUrlPrefix: true,
+  updateIconBadge: true,
+  styleViaAPI: true,
+}, {
   get: (target, name) =>
-    (...args) => Promise.resolve(self.msg.sendBg({
+    (...args) => Promise.resolve(self.msg[target[name] ? 'send' : 'sendBg']({
       method: 'invokeAPI',
       name,
       args