Merge pull request #7597 from minrk/position-header

apply `position: relative` to header

IPython/html/static/base/less/page.less

@@ -23,6 +23,7 b' body {'
     background-color: @body-bg;
 
     /* Display over codemirror */
+    position: relative;
     z-index: 100;
 
     #header-container {

IPython/html/static/style/style.min.css

@@ -8401,6 +8401,7 b' body {'
   display: none;
   background-color: #ffffff;
   /* Display over codemirror */
+  position: relative;
   z-index: 100;
 }
 #header #header-container {