hterm: disable default handling for mouse buttons 3 & 4

These usually cause the browser to navigate forward/backward which
kills the ssh session.  Disable their default handling.

Change-Id: I25b207eb679659152165bd54629108ad995c22fd
Reviewed-on: https://chromium-review.googlesource.com/1194585
Reviewed-by: Vitaliy Shipitsyn <vsh@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/hterm/js/hterm_terminal.js b/hterm/js/hterm_terminal.js
index 8f55b55..09497ad 100644
--- a/hterm/js/hterm_terminal.js
+++ b/hterm/js/hterm_terminal.js
@@ -3416,6 +3416,13 @@
     return;
   }
 
+  // Consume navigation events.  Button 3 is usually "browser back" and
+  // button 4 is "browser forward" which we don't want to happen.
+  if (e.button > 2) {
+    e.preventDefault();
+    // We don't return so click events can be passed to the remote below.
+  }
+
   var reportMouseEvents = (!this.defeatMouseReports_ &&
       this.vt.mouseReport != this.vt.MOUSE_REPORT_DISABLED);