Lower z-indexes

Canvas layers: 0-4
Helpers: 5
A11y tree: 10

Fixes #2275

css/xterm.css

@@ -55,7 +55,7 @@
      * The z-index of the helpers must be higher than the canvases in order for
      * IMEs to appear on top.
      */
-    z-index: 10;
+    z-index: 5;
 }
 
 .xterm .xterm-helper-textarea {
@@ -69,7 +69,7 @@
     top: 0;
     width: 0;
     height: 0;
-    z-index: -10;
+    z-index: -5;
     /** Prevent wrapping so the IME appears against the textarea at the correct position */
     white-space: nowrap;
     overflow: hidden;
@@ -150,7 +150,7 @@
     top: 0;
     bottom: 0;
     right: 0;
-    z-index: 100;
+    z-index: 10;
     color: transparent;
 }