Move tooltips to position fixed

res/style.css

@@ -31,8 +31,6 @@ body {
 }
 
 #root {
-  /* Create a stacking context so that tooltips can be placed over the root element. */
-  position: relative;
   z-index: 0;
 }
 

src/content/components/TimelineCanvas.js

@@ -13,7 +13,7 @@ type Props<HoveredItem> = {
   isDragging: boolean,
   getHoveredItemInfo: HoveredItem => React$Element<*>,
   drawCanvas: (CanvasRenderingContext2D, HoveredItem | null) => void,
-  onDoubleClickItem: HoveredItem => void,
+  onDoubleClickItem: (HoveredItem | null) => void,
   hitTest: (x: CssPixels, y: CssPixels) => HoveredItem | null,
 };
 

src/content/components/Tooltip.css

@@ -1,17 +1,11 @@
 .tooltipMount {
-  /* Create a stacking context above the root container element. */
-  position: absolute;
-  z-index: 1;
-  top: 0;
-  left: 0;
-  width: 100%;
-  height: 100%;
-  overflow: hidden;
   pointer-events: none;
+  /* Stack the tooltip mounting point above the root element. */
+  z-index: 1;
 }
 
 .tooltip {
-  position: relative;
+  position: fixed;
   max-width: 100%;
   box-sizing: border-box;
   display: inline-block;