Use tab-change event handler

src/background/tabs.js

@@ -1,5 +1,13 @@
-let prevSelTab = 0;
+let prevSelTab = 1;
-let currSelTab = 2;
+let currSelTab = 1;
+
+browser.tabs.onActivated.addListener(tabChangeHandler);
+
+function tabChangeHandler(activeInfo) {
+  prevSelTab = currSelTab;
+  currSelTab = activeInfo.tabId;
+  console.log("prev tab: " + prevSelTab + " - curr tab: " + currSelTab);
+}
 
 const closeTab = (id) => {
   return browser.tabs.remove(id);
@@ -71,6 +79,7 @@ const selectPrevTab = (current, count) => {
     }
     let select = (current - count + tabs.length) % tabs.length;
     let id = tabs[select].id;
+    currSelTab = id;
     return browser.tabs.update(id, { active: true });
   });
 };
@@ -86,6 +95,7 @@ const selectNextTab = (current, count) => {
     }
     let select = (current + count) % tabs.length;
     let id = tabs[select].id;
+    currSelTab = id;
     return browser.tabs.update(id, { active: true });
   });
 };
@@ -97,6 +107,7 @@ const selectFirstTab = () => {
 
   return browser.tabs.query({ currentWindow: true }).then((tabs) => {
     let id = tabs[0].id;
+    currSelTab = id;
     return browser.tabs.update(id, { active: true });
   });
 };
@@ -108,6 +119,7 @@ const selectLastTab = () => {
 
   return browser.tabs.query({ currentWindow: true }).then((tabs) => {
     let id = tabs[tabs.length - 1].id;
+    currSelTab = id;
     return browser.tabs.update(id, { active: true });
   });
 };