rollback #3358

closes  #3391

For users who need to hide the scrollbars while zooming text in the meantime, you can add the following to your custom.css:

```css
.CodeMirror-scroll, .CodeMirror-wrap .CodeMirror-scroll {
  overflow: scroll;
}

.CodeMirror-vscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-hscrollbar {
    visibility: hidden;
}
```

IPython/frontend/html/notebook/static/notebook/less/codemirror.less

@@ -18,22 +18,8 b''
 .CodeMirror-scroll {
     /*  The CodeMirror docs are a bit fuzzy on if overflow-y should be hidden or visible.*/
     /*  We have found that if it is visible, vertical scrollbars appear with font size changes.*/
-    /*  overflow: scroll prevents scrollbars being drawn on top of the last line of the cell, */
-    /*  but at the expense of extra gutter at the bottom. To compensate, we shrink the margin */
-    /*  between the last line and the scrollbar when it's drawn. */
-    overflow: scroll;
-    margin-bottom: -38px; /* default margin is -30px */
-}
-
-/* this one is needed for Text Cells to match code cells for some reason */
-.CodeMirror-wrap .CodeMirror-scroll {
-  overflow: scroll;
-}
-
-/* CM sometimes draws a vertical scrollbar under zoom, but it never should in the Notebook. */
-/* this hides them - display: none would be preferable, but it gets clobbered by CodeMirror */
-.CodeMirror-vscrollbar, .CodeMirror-scrollbar-filler {
-    visibility: hidden;
+    overflow-y: hidden;
+    overflow-x: auto;
 }
 
 .CodeMirror-lines {

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

@@ -940,9 +940,7 b' div.out_prompt_overlay{height:100%;padding:0px;position:absolute;border-radius:4'
 div.out_prompt_overlay:hover{-webkit-box-shadow:inset 0 0 1px #000000;-moz-box-shadow:inset 0 0 1px #000000;box-shadow:inset 0 0 1px #000000;background:rgba(240, 240, 240, 0.5);}
 div.output_prompt{color:darkred;margin:0 5px 0 -5px;}
 .CodeMirror{line-height:1.231;height:auto;background:none;}
-.CodeMirror-scroll{overflow:scroll;margin-bottom:-38px;}
-.CodeMirror-wrap .CodeMirror-scroll{overflow:scroll;}
-.CodeMirror-vscrollbar,.CodeMirror-scrollbar-filler{visibility:hidden;}
+.CodeMirror-scroll{overflow-y:hidden;overflow-x:auto;}
 .CodeMirror-lines{padding:0.4em;}
 .CodeMirror-linenumber{padding:0 8px 0 4px;}
 .CodeMirror-gutters{border-bottom-left-radius:4px;border-top-left-radius:4px;}