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

go-back-with-backspace/background.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.
 
-// Put up an informative message on first install.
+// Inject the content scripts into all open tabs on first install or update.
 chrome.runtime.onInstalled.addListener(function(details) {
-  if (details.reason == "install") {
-    chrome.tabs.create({url: "pages/installed.html"});
-  }
+  if (details.reason == 'install' || details.reason === 'update')
+    injectContentScripts();
 });
+
+// Inject the content scripts into all open tabs when this extension is
+// re-enabled.
+chrome.management.onEnabled.addListener(function(info) {

[Devlin, 2016/09/19 21:22:31]

(Just thinking) Hmm... I wonder if we should make a runtime event for this.

[Pam (message me for reviews), 2016/09/20 21:42:59]

I'd star that bug. Perhaps also onDisabled, since onSuspend doesn't seem to
cover that.

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

(Orthogonal, but FYI) onDisabled wouldn't make sense, because when the extension
is unloaded, we require that it stop running (synchronously).  Having an
onDisabled event would break that contract.

+  if (info.id === chrome.runtime.id)
+    injectContentScripts();
+});
+
+// Maintain a longstanding connection to the content script, so the old version
+// can disable itself when the extension has been updated, disabled, or
+// uninstalled.

[Devlin, 2016/09/19 21:22:31]

This is kind of an abuse of the API.  It means we'll not only need to keep this
port open forever, but also the background page in general - constantly
consuming resources.

In general, chrome makes no effort to clean up content scripts when an extension
has been removed.  I don't think this one should be any different.

If you really want to keep this functionality, the other option would be to do a
ping from the content script when backspace is triggered (which should be
relatively rare).

[Pam (message me for reviews), 2016/09/20 21:42:59]

Chrome can't really clean up content scripts, but judging from the various forum
discussions, some do go to the effort. I'd like to keep the nicer behavior.

I was trying to avoid the extension having to wait for a response or error on
every backspace, and thus being slow. It seems OK, though, so I'll wait and see
if anyone complains.

Note that there is an oddity revealed with the extension system. When the
extension is disconnected (e.g. disabled), runtime.sendMessage throws an error
about the arguments not matching the schema. I spent a day digging into it, then
gave up and just catch it.

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

Can you file a bug for that?

+chrome.runtime.onConnect.addListener(function(port) {});
+
+function injectContentScripts() {
+  var scripts = chrome.runtime.getManifest().content_scripts[0].js;

[Devlin, 2016/09/19 21:22:31]

given the fact that you're already relying on manifest structure here, I might
just define the file names and save an API call...

[Pam (message me for reviews), 2016/09/20 21:42:59]

True, although the manifest structure is likely to change a lot less often than
the manifest contents. I'm inclined to keep it more robust against the developer
forgetting to update the list. Especially since I'm that potentially forgetful
developer -- but even more if anybody else ever works on the thing.

+
+  chrome.windows.getAll({populate: true}, function(windows) {

[Devlin, 2016/09/19 21:22:31]

this would be easier with chrome.tabs.query({}, tabs => {
})

[Pam (message me for reviews), 2016/09/20 21:42:59]

Done, thanks (except that I used function() syntax to be consistent with the
rest of the file).

+    for (var window_index = 0; window_index < windows.length; ++window_index) {
+      var win = windows[window_index];
+      for (var tab_index = 0; tab_index < win.tabs.length; ++tab_index) {
+        var tab = win.tabs[tab_index];
+        for (var i = 0; i < scripts.length; ++i) {
+          chrome.tabs.executeScript(tab.id,
+                                    {
+                                      file: scripts[i],
+                                      allFrames: true,
+                                      runAt: 'document_start'

[Devlin, 2016/09/19 21:22:31]

Why document start?

[Pam (message me for reviews), 2016/09/20 21:42:59]

A number of users want to be able to hit back-back-back and go back three pages
in quick succession, as they could when it was in the browser. The faster we can
get up and listening to the keyboard, the better.

+                                    });
+        }
+      }
+    }
+  });
+}