Fix missing cell popover opacity transition

+ flash of content on initial load for cell contents being limited by the new inline max-width

src/components/datagrid/_data_grid_data_row.scss

@@ -111,6 +111,12 @@
   // since we don't use the popover arrow in any case for cell popovers
   filter: none;
   @include euiBottomShadow; // TODO: Convert to euiShadowMedium() in Emotion
+
+  // For some reason, the normal popover opacity transition doesn't work for datagrid popovers
+  // so we'll force it via an animation. If we don't, cells constrained by the inline max-width
+  // style that we set will see a flash of unwanted content before repositioning
+  animation-duration: $euiAnimSpeedNormal;
+  animation-name: euiDataGridCellPopover;
 }
 
 .euiDataGridRowCell--controlColumn .euiDataGridRowCell__content {
@@ -253,3 +259,12 @@
     transform: scaleY(1);
   }
 }
+
+@keyframes euiDataGridCellPopover {
+  from {
+    opacity: 0;
+  }
+  to {
+    opacity: 1;
+  }
+}