Make sure that the default value of the hidden input field is a space.

include/ui.js

@@ -854,7 +854,7 @@ keyInput: function(event) {
     // another session there has to always be text in the
     // keyboardinput element with which backspace can interact.
     // We also need to reset the input field text to avoid overflow.
-    elem.value = "x";
+    elem.value = '\u00a0';
 },
 
 keyInputBlur: function() {

vnc.html

@@ -65,7 +65,7 @@
                     id="showKeyboard" class="noVNC_status_button"
                     value="Keyboard" title="Show Keyboard"/>
                 <input type="text" autocapitalize="off" autocorrect="off"
-                    id="keyboardinput" class=""/>
+                    id="keyboardinput" class="" value="&nbsp;"/>
                 <div id="noVNC_extra_keys">
                     <input type="image" alt="Extra keys" src="images/showextrakeys.png"
                         id="showExtraKeysButton" class="noVNC_status_button">