Add spellcheck

As of Electron 1.6.5, this requires disabling the sandbox in order to
get access to the `webFrame` api.

// FREEBIE

main.js

@@ -73,7 +73,7 @@ function createWindow () {
     height: 610,
     webPreferences: {
       nodeIntegration: false,
-      sandbox: true,
+      //sandbox: true,
       preload: path.join(__dirname, 'preload.js')
     }
   })

package.json

@@ -98,6 +98,8 @@
   },
   "dependencies": {
     "config": "^1.25.1",
+    "electron-editor-context-menu": "^1.1.1",
+    "electron-spell-check-provider": "^1.1.0",
     "electron-updater": "^1.11.2"
   }
 }

preload.js

@@ -8,4 +8,60 @@
   const ipc = electron.ipcRenderer
   window.config.locale_json = ipc.sendSync('locale-data');
 
+  /**
+  * Enables spell-checking and the right-click context menu in text editors.
+  * Electron (`webFrame.setSpellCheckProvider`) only underlines misspelled words;
+  * we must manage the menu ourselves.
+  *
+  * Run this in the renderer process.
+  */
+  var remote = electron.remote;
+  var webFrame = electron.webFrame;
+  var SpellCheckProvider = require('electron-spell-check-provider');
+  // `remote.require` since `Menu` is a main-process module.
+  var buildEditorContextMenu = remote.require('electron-editor-context-menu');
+
+  var selection;
+  function resetSelection() {
+    selection = {
+      isMisspelled: false,
+      spellingSuggestions: []
+    };
+  }
+  resetSelection();
+
+  // Reset the selection when clicking around, before the spell-checker runs and the context menu shows.
+  window.addEventListener('mousedown', resetSelection);
+
+  // The spell-checker runs when the user clicks on text and before the 'contextmenu' event fires.
+  // Thus, we may retrieve spell-checking suggestions to put in the menu just before it shows.
+  webFrame.setSpellCheckProvider(
+    'en-US',
+    // Not sure what this parameter (`autoCorrectWord`) does: https://github.com/atom/electron/issues/4371
+    // The documentation for `webFrame.setSpellCheckProvider` passes `true` so we do too.
+    true,
+    new SpellCheckProvider('en-US').on('misspelling', function(suggestions) {
+      // Prime the context menu with spelling suggestions _if_ the user has selected text. Electron
+      // may sometimes re-run the spell-check provider for an outdated selection e.g. if the user
+      // right-clicks some misspelled text and then an image.
+      if (window.getSelection().toString()) {
+        selection.isMisspelled = true;
+        // Take the first three suggestions if any.
+        selection.spellingSuggestions = suggestions.slice(0, 3);
+      }
+    }));
+
+  window.addEventListener('contextmenu', function(e) {
+    // Only show the context menu in text editors.
+    if (!e.target.closest('textarea, input, [contenteditable="true"]')) return;
+
+    var menu = buildEditorContextMenu(selection);
+
+    // The 'contextmenu' event is emitted after 'selectionchange' has fired but possibly before the
+    // visible selection has changed. Try to wait to show the menu until after that, otherwise the
+    // visible selection will update after the menu dismisses and look weird.
+    setTimeout(function() {
+      menu.popup(remote.getCurrentWindow());
+    }, 30);
+  });
 })();