A tile that contains a vacuum has no tooltip when hovered in some overlays. In the regular view, hovering the mouse pointer over a tile that contains a vacuum will display the vacuum tooltip. However, in Oxygen, Materials, Plumbing, and Ventilation overlays give no tooltip at all while hovering over the vacuum tile. The thermal and electrical overlays do have the tooltip.
Produce a tile that contains no gas; diagonal dig, with a pump, digging on the 'other side' of water Hover mouse pointer over tile containing vacuum Cycle though overlay modes