"slab" hides "echo" text

When "slab" is activated, the "echo" text disappears.
Upon "slab off", it shows up again.

It looks like the echo text is somewhere on
Z-coordinate. Miguel said it is at Z=0. However,
**any** value after "slab" hides the text (e.g. slab 1,
slab 99). Only slab off allows to see the text.