Inject automatically on install etc., add mime types, and fix nits.

go-back-with-backspace/content_script.js

 // Copyright 2016 Google Inc. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Load the options, then register a keyboard listener.  We capture the event
 // at the Window to let any handlers or listeners registered on the Document
 // have a chance to handle it first.
 var options;
 chrome.storage.sync.get({
   blacklist: [],
   disableInApplets: true,
   whitelist: []
 }, function(items) {
   options = items;
-  window.addEventListener('keydown', function(e) {
-    handleBackspace(e);
-  });
+  window.addEventListener('keydown', handleBackspace);
 });
 
 // Update the local options when they're changed externally.
 chrome.storage.onChanged.addListener(function(changes, area) {
   if (area === 'sync') {
     if (changes.blacklist)
       options.blacklist = changes.blacklist.newValue;
     if (changes.disableInApplets)
       options.disableInApplets = changes.disableInApplets.newValue;
     if (changes.whitelist)
       options.whitelist = changes.whitelist.newValue;
   }
 });
 
 // Check for shift-backspace or unmodified backspace and navigate if
 // applicable.
 function handleBackspace(e) {
   if (e.defaultPrevented ||
       e.key !== 'Backspace' ||
       e.altKey ||
       e.ctrlKey ||
-      e.metaKey)
+      e.metaKey ||
+      window.history.length < 2) // Nowhere to go back or forward to anyway.
     return;
 
   // The blacklist overrides everything.
   var url = window.location.href;
   if (options.blacklist.includes(url))
     return;
 
   // The whitelist overrides applet focus.
   // Listening on the Window means the event has no path (see
   // http://crbug.com/645527), so we'll have to look at the focused (active)
   // element. This means it will not work properly with shadow DOM.
   // TODO: Fix behavior with shadow DOM when the above bug is resolved.
   if (!options.whitelist.includes(url) &&
       disabledInApplet(document.activeElement))
     return;
   if (isEditable(document.activeElement))
     return;
 
-  e.shiftKey ? window.history.forward(): window.history.back();
-  e.preventDefault();
+  // Make sure this extension is still active.
+  // sendMessage throws an internal error, not reported in lastError, if the
+  // other end no longer exists. So we use JS error-catching rather than
+  // extension errors.

[Devlin, 2016/09/27 16:08:57]

please add a crbug.com/nnn here.

[Pam (message me for reviews), 2016/09/27 23:37:32]

I'll take that as confirmation that it's not intended behavior. Done.

+  try {
+    chrome.runtime.sendMessage('', function(response) {

[Devlin, 2016/09/27 16:08:57]

shouldn't need ''

[Pam (message me for reviews), 2016/09/27 23:37:32]

That's the message, which is not optional. If it should be, let me know and I'll
file a bug for it.

[Devlin, 2016/09/29 20:55:34]

Whoops, you're right.  This is fine.

+      // Future-proofing in case sendMessage ever changes to setting lastError
+      // instead of throwing a JS error.
+      if (chrome.runtime.lastError) {
+        window.removeEventListener('keydown', handleBackspace);
+      } else {
+        e.shiftKey ? window.history.forward(): window.history.back();
+        e.preventDefault();
+      }
+    });
+  } catch(error) {
+    // If we have no connection to the background page, the extension has
+    // been updated, disabled, or uninstalled. Remove our listener and do
+    // nothing.
+    window.removeEventListener('keydown', handleBackspace);
+  }
 }
 
 // Return true if the option to disable the extension in applets is enabled,
 // and focus is in an embedded Flash or Java applet.
 function disabledInApplet(target) {
   if (!options.disableInApplets)
     return false;
 
   var nodeName = target.nodeName.toUpperCase();
   var nodeType = target.type || '';
   nodeType = nodeType.toLowerCase();
   if ((nodeName === 'EMBED' || nodeName === 'OBJECT') &&
-      (nodeType === 'application/x-shockwave-flash' ||
-       nodeType === 'application/java')) {
+      (nodeType === 'application/java' ||
+       nodeType === 'application/pdf' ||
+       nodeType === 'application/x-chat' ||
+       nodeType === 'application/x-google-chrome-pdf' ||
+       nodeType === 'application/x-shockwave-flash')) {
     return true;
   }
   return false;
 }