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diff --git a/content_scripts/mode_key_handler.coffee b/content_scripts/mode_key_handler.coffee
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+
+# The important data structure here is the "keyState".  The key state is a non-empty list of objects, the keys
+# of which are key names, and the values are other key-mapping objects or commands (strings).  Key-mapping
+# objects can be arbitrarily nested; so we support any length of multi-key mapping.
+#
+# Whenever we consume a key, we append a new copy of the global key mapping to the key state (hence, the
+# global mappings are always available, and the key state is always non-empty).
+
+class KeyHandlerMode extends Mode
+  useCount: true
+  countPrefix: 0
+  keydownEvents: {}
+  keyState: []
+
+  constructor: (options) ->
+    # A function accepting a command name and a count; required.
+    @commandHandler = options.commandHandler ? (->)
+    # A Key mapping structure; required.
+    @keyMapping = options.keyMapping ? {}
+    @useCount = false if options.noCount
+    @reset()
+
+    # We don't pass these options on to super().
+    options = Utils.copyObjectOmittingProperties options, "commandHandler", "keyMapping", "noCount"
+
+    super extend options,
+      keydown: @onKeydown.bind this
+      keypress: @onKeypress.bind this
+      keyup: @onKeyup.bind this
+      # We cannot track matching keydown/keyup events if we lose the focus.
+      blur: (event) => @alwaysContinueBubbling =>
+        @keydownEvents = {} if event.target == window
+
+  onKeydown: (event) ->
+    keyChar = KeyboardUtils.getKeyCharString event
+
+    if KeyboardUtils.isEscape event
+      if @isInResetState()
+        @continueBubbling
+      else
+        @reset()
+        DomUtils.suppressKeyupAfterEscape handlerStack
+        false # Suppress event.
+
+    else if keyChar and @keyCharIsKeyStatePrefix keyChar
+        @advanceKeyState keyChar
+        commands = @keyState.filter (entry) -> "string" == typeof entry
+        @invokeCommand commands[0] if 0 < commands.length
+        false # Suppress event.
+
+    else
+      # We did not handle the event, but we might handle the subsequent keypress event.  If we *will* be
+      # handling that event, then we need to suppress propagation of this keydown event to prevent triggering
+      # page features like Google instant search.
+      keyChar = KeyboardUtils.getKeyChar event
+      if keyChar and (@keyCharIsKeyStatePrefix(keyChar) or @isCountKey keyChar)
+        DomUtils.suppressPropagation event
+        @keydownEvents[@getEventCode event] = true
+        @stopBubblingAndTrue
+      else
+        @countPrefix = 0 if keyChar
+        @continueBubbling
+
+  onKeypress: (event) ->
+    keyChar = KeyboardUtils.getKeyCharString event
+    if keyChar and @keyCharIsKeyStatePrefix keyChar
+      @advanceKeyState keyChar
+      commands = @keyState.filter (entry) -> "string" == typeof entry
+      @invokeCommand commands[0] if 0 < commands.length
+      false # Suppress event.
+    else if keyChar and @isCountKey keyChar
+      @countPrefix = @countPrefix * 10 + parseInt keyChar
+      false # Suppress event.
+    else
+      @continueBubbling
+
+  onKeyup: (event) ->
+    eventCode = @getEventCode event
+    if eventCode of @keydownEvents
+      delete @keydownEvents[eventCode]
+      DomUtils.suppressPropagation event
+      @stopBubblingAndTrue
+    else
+      @continueBubbling
+
+  # This tests whether keyChar is a prefix of any current mapping in the key state.
+  keyCharIsKeyStatePrefix: (keyChar) ->
+    for mapping in @keyState
+      return true if keyChar of mapping
+    false
+
+  # This is called whenever a keyChar is matched.  We keep any existing entries matching keyChar, and append a
+  # new copy of the global key mappings.
+  advanceKeyState: (keyChar) ->
+    newKeyState =
+      for mapping in @keyState
+        continue unless keyChar of mapping
+        mapping[keyChar]
+    @keyState = [newKeyState..., @keyMapping]
+
+  # This is called to invoke a command and reset the key state.
+  invokeCommand: (command) ->
+    countPrefix = if 0 < @countPrefix then @countPrefix else 1
+    @reset()
+    @commandHandler command, countPrefix
+
+  # Reset the state (as if no keys had been handled).
+  reset: ->
+    @countPrefix = 0
+    @keyState = [@keyMapping]
+
+  # This tests whether we are in the reset state.  It is used to check whether we should be using escape to
+  # reset the key state, or passing it to the page.
+  isInResetState: ->
+    @countPrefix == 0 and @keyState.length == 1
+
+  # This tests whether keyChar should be treated as a count key.
+  isCountKey: (keyChar) ->
+    return false unless @useCount and keyChar.length == 1
+    if 0 < @countPrefix
+      '0' <= keyChar <= '9'
+    else
+      '1' <= keyChar <= '9'
+
+  getEventCode: (event) -> event.keyCode
+
+# Demo/test code.
+# A (very) poor-man's normal mode.
+
+demoKeyMapping =
+  j: "scrollDown"
+  k: "scrollUp"
+  i: "enterInsertMode"
+  g:
+    g: "scrollToTop"
+    a: "scrollToTop"
+    z: "scrollToBottom"
+    i: "focusInput"
+  # A three-key binding.
+  a:
+    b:
+      c: "enterInsertMode"
+      # And this should override "j" on its own.
+      j: "enterInsertMode"
+
+demoCommandHandler = (command, count) ->
+  switch command
+    when "scrollDown" then scrollDown()
+    when "scrollUp" then scrollUp()
+    when "scrollToTop" then scrollToTop count
+    when "scrollToBottom" then scrollToBottom()
+    when "enterInsertMode" then enterInsertMode()
+    when "focusInput" then focusInput count
+
+root = exports ? window
+root.KeyHandlerMode = KeyHandlerMode
+root.demoKeyMapping = demoKeyMapping
+root.demoCommandHandler = demoCommandHandler