Set parent width for coverGutter widgets in measure div

When using the measure div to measure a LineWidget with coverGutter: true,
set the parent's width to include the gutter. Otherwise, the widget will
be measured as if it were not meant to cover the gutter (it will be
measured at a narrower width).

Fixes #2611

Author: Adam Ahmed

lib/codemirror.js

@@ -5541,8 +5541,12 @@
 
   function widgetHeight(widget) {
     if (widget.height != null) return widget.height;
-    if (!contains(document.body, widget.node))
-      removeChildrenAndAdd(widget.cm.display.measure, elt("div", [widget.node], null, "position: relative"));
+    if (!contains(document.body, widget.node)) {
+      var parentStyle = "position: relative;";
+      if (widget.coverGutter)
+        parentStyle += "margin-left: -" + widget.cm.getGutterElement().offsetWidth + "px;";
+      removeChildrenAndAdd(widget.cm.display.measure, elt("div", [widget.node], null, parentStyle));
+    }
     return widget.height = widget.node.offsetHeight;
   }