Change z-index of ipython_tooltip

I experienced the problem stated in  #5755. Giving the ipython_tooltip class a higher z-index indeed solves the problem. I intentionally set it to a high value so it likelier remains on top when styles are changed in the future.

IPython/html/static/notebook/less/tooltip.less

@@ -1,158 +1,158
 /**
  * Primary styles
  *
  * Author: IPython Development Team
  */
 
 /** WARNING IF YOU ARE EDITTING THIS FILE, if this is a .css file, It has a lot
  * of chance of beeing generated from the ../less/[samename].less file, you can
  * try to get back the less file by reverting somme commit in history
  **/
 
 /*
  * We'll try to get something pretty, so we
  * have some strange css to have the scroll bar on
  * the left with fix button on the top right of the tooltip
  */
 
 // double slash comment are remove by less compilation
 // **
 // * Less mixins
 // **/
 
 // Four color of the background
 @import "variables" ;
 
 .dropshadow(){
  -moz-box-shadow:    0px 6px 10px -1px #adadad;
   -webkit-box-shadow: 0px 6px 10px -1px #adadad;
   box-shadow:         0px 6px 10px -1px #adadad;
 }
 
 // smoth height adaptation
 .smoothheight(@t:500ms) {
     -webkit-transition-property: height;
     -webkit-transition-duration: @t;
     -moz-transition-property: height;
     -moz-transition-duration: @t;
     transition-property: height;
     transition-duration: @t;
 }
 
 @-moz-keyframes fadeOut {
     from {opacity:1;}
     to {opacity:0;}
 }
 
 @-webkit-keyframes fadeOut {
     from {opacity:1;}
     to {opacity:0;}
 }
 
 @-moz-keyframes fadeIn {
     from {opacity:0;}
     to {opacity:1;}
 }
 
 @-webkit-keyframes fadeIn {
     from {opacity:0;}
     to {opacity:1;}
 }
 
 /*properties of tooltip after "expand"*/
 .bigtooltip {
     overflow: auto;
     height: 200px;
     .smoothheight();
 }
 
 /*properties of tooltip before "expand"*/
 .smalltooltip{
     .smoothheight();
     text-overflow: ellipsis;
     overflow: hidden;
     height:80px;
 }
 
 .tooltipbuttons
 {
     position: absolute;
     padding-right : 15px;
     top : 0px;
     right:0px;
 }
 
 .tooltiptext
 {
     /*avoid the button to overlap on some docstring*/
     padding-right:30px
 }
 
 .ipython_tooltip {
     max-width:700px;
     /*fade-in animation when inserted*/
     -webkit-animation: fadeOut 400ms;
     -moz-animation: fadeOut 400ms;
     animation: fadeOut 400ms;
     -webkit-animation: fadeIn 400ms;
     -moz-animation: fadeIn 400ms;
     animation: fadeIn 400ms;
     vertical-align: middle;
     background-color: @cell_background;
 
     overflow : visible;
     border: @border_color @border_width solid;
     outline: none;
     padding: 3px;
     margin: 0px;
     padding-left:7px;
     font-family: @monoFontFamily;
     min-height:50px;
 
     .dropshadow;
     .corner-all;
 
     a {
         float:right;
     };
     position: absolute;
 
-	z-index: 2;
+	z-index: 1000;
 	
 	.tooltiptext {
 		pre {
 			border: 0;
 			.border-radius(0);
 			font-size: 100%;
 			background-color: @cell_background;
 		}
 	}
 }
 
 .pretooltiparrow {
   left: 0px;
   margin: 0px;
   top: -16px;
   width: 40px;
   height: 16px;
   overflow: hidden;
   position: absolute;
 
 }
 
 .pretooltiparrow:before {
   background-color : @cell_background;
   border : @border_width @border_color solid;
   z-index:11;
   content: "";
   position: absolute;
   left: 15px;
   top: 10px;
   width: 25px;
   height: 25px;
   @theta : 45deg;
   -webkit-transform: rotate(@theta);
   -moz-transform:    rotate(@theta);
   -ms-transform:     rotate(@theta);
   -o-transform:      rotate(@theta);
 }