Don't use position: fixed in context-menu hack

It doesn't work inside of a transformed container, even for zero
transforms (which some components seem to love putting on root
elements, for whichever reason)

Closes #3238

lib/codemirror.js

@@ -1482,10 +1482,11 @@
       if (reset && cm.doc.sel.contains(pos) == -1)
         operation(cm, setSelection)(cm.doc, simpleSelection(pos), sel_dontScroll);
 
-      var oldCSS = te.style.cssText;
-      input.wrapper.style.position = "absolute";
-      te.style.cssText = "position: fixed; width: 30px; height: 30px; top: " + (e.clientY - 5) +
-        "px; left: " + (e.clientX - 5) + "px; z-index: 1000; background: " +
+      var oldCSS = te.style.cssText, oldWrapperCSS = input.wrapper.style.cssText;
+      input.wrapper.style.cssText = "position: absolute"
+      var wrapperBox = input.wrapper.getBoundingClientRect()
+      te.style.cssText = "position: absolute; width: 30px; height: 30px; top: " + (e.clientY - wrapperBox.top - 5) +
+        "px; left: " + (e.clientX - wrapperBox.left - 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)
@@ -1516,7 +1517,7 @@
       }
       function rehide() {
         input.contextMenuPending = false;
-        input.wrapper.style.position = "relative";
+        input.wrapper.style.cssText = oldWrapperCSS
         te.style.cssText = oldCSS;
         if (ie && ie_version < 9) display.scrollbars.setScrollTop(display.scroller.scrollTop = scrollPos);