In TAP Search panel, loading masks for Spatial/Temporal area and Column Constraints area prevent user from selecting another schema or table, while loading mask is displayed.

If loading-mask css class did not set z-index, the components would be displayed correctly, and select drop-down would appear on top of the loading mask.

Whenever possible, we should avoid using z-index, so that our components are following the natural stacking order, described in the link below:

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/Stacking_without_z-index