0 is a valid tab id

background/usercss-helper.js

@@ -85,7 +85,8 @@ var usercssHelper = (() => {
     return wrapReject(openURL({
       url: '/install-usercss.html' +
         '?updateUrl=' + encodeURIComponent(url) +
-        '&tabId=' + (direct ? -tab.id : tab.id),
+        '&tabId=' + tab.id +
+        (direct ? '&direct=yes' : ''),
       index: tab.index + 1,
       openerTabId: tab.id,
     }));

install-usercss/install-usercss.js

@@ -10,7 +10,7 @@
   const tabId = Number(params.get('tabId'));
   let port;
 
-  if (tabId < 0) {
+  if (params.has('direct')) {
     $('.live-reload').remove();
     getCodeDirectly();
   } else {
@@ -327,7 +327,7 @@
   function getCodeDirectly() {
     // FF applies page CSP even to content scripts, https://bugzil.la/1267027
     // To circumvent that, the bg process downloads the code directly
-    const key = 'tempUsercssCode' + (-tabId);
+    const key = 'tempUsercssCode' + tabId;
     chrome.storage.local.get(key, data => {
       const code = data && data[key];
 

js/messaging.js

@@ -128,7 +128,7 @@ function notifyAllTabs(msg) {
   }
   // notify background page and all open popups
   if (affectsSelf) {
-    msg.tabId = null;
+    msg.tabId = undefined;
     sendMessage(msg, ignoreChromeError);
   }
 }
@@ -144,8 +144,8 @@ function sendMessage(msg, callback) {
     - enabled by passing a second param
   */
   const {tabId, frameId} = msg;
-  const fn = tabId ? chrome.tabs.sendMessage : chrome.runtime.sendMessage;
-  const args = tabId ? [tabId, msg, {frameId}] : [msg];
+  const fn = tabId >= 0 ? chrome.tabs.sendMessage : chrome.runtime.sendMessage;
+  const args = tabId >= 0 ? [tabId, msg, {frameId}] : [msg];
   if (callback) {
     fn(...args, callback);
   } else {