Force stable y scroll when focusing textarea in onContextMenu

Closes #2712

Author: marijnh

lib/codemirror.js

@@ -3319,7 +3319,9 @@
       "px; left: " + (e.clientX - 5) + "px; z-index: 1000; background: " +
       (ie ? "rgba(255, 255, 255, .05)" : "transparent") +
       "; outline: none; border-width: 0; outline: none; overflow: hidden; opacity: .05; filter: alpha(opacity=5);";
+    if (webkit) var oldScrollY = window.scrollY; // Work around Chrome issue (#2712)
     focusInput(cm);
+    if (webkit) window.scrollTo(null, oldScrollY);
     resetInput(cm);
     // Adds "Select all" to context menu in FF
     if (!cm.somethingSelected()) display.input.value = display.prevInput = " ";