Thought it wasn't working at first, but then I went into spectator mode and realized that the rectangle being rendered only goes from -64 to -0

The slime chunk overlay renders from the bottom of the world up to the height where your player is at when you toggle it on. And you can also adjust the top height if you hold down the toggle hotkey and scroll with the mouse. However now that I checked it, seems like I derped that top y position setter - it only sets the top y position if you use the hotkey to turn it on, not if you turn it on via the config menu. So I'll need to fix that bit.

This "wasn't an issue" in older versions of the mod, because the config menu was bad and didn't have the direct boolean toggle, so it was only possible to turn it on via the hotkey...

Just a bump because I also ran into this problem. Started digging out at Y0 at first because I trusted the render.

Seems strange to have the y value customizable. Slimes only spawn up to a certain y level (which I can't remember). It would be helpful if the overlay conveyed that information unless the player goes out of their way to change it to something else

@lbenedetto The overlay is more meant for locating and seeing the chunks themselves, not for seeing the actual spawn height range. Especially in survival the overlay would be pretty useless for finding suitable slime chunks (for example by elytra flying) if the overlay was only deep below ground.

But I could make it a config option that it would show the actual spawn height range (that would be assuming the server doesn't have a mod that changes the behavior from vanilla...).

When I switched on the slime chunk renderer (by GUI button), I expected it to be full world height. When it wasn't, I assumed it would at the slime spawn height--because what else would it be.

As an adjustable value, it would also be nice if it appeared in the config GUI, presumably under Generics. With a tooltip mentioning slime spawn level or a button to set it to that value? I would only ever find the hotkey/scroll wheel adjustment by accident.

Failing all that, setting it to current player y when toggled by GUI button (same as hotkey) would at least be consistent and initially visible.

@JimmXinu The config tooltips in general are pretty bad and severely lacking in details and information in the current mod releases in 1.13+. I've already spent literally at least 10 hours going through and improving them for most of the mods in the 1.12.2 development code. I think Tweakeroo's tooltips are still to-do.

And I can also add a message about setting the top y position to the player's position if/when it happens, meaning that I'm likely going to add that new option to select if/where it should get set.

And it not getting set to the player's position when you toggle the overlay via the GUI button is a bug. Previously there didn't use to be direct toggle buttons at all so the callback that sets it to the player's position was forgotten to be moved, and it still only happens from the hotkey toggle, not from the underlying boolean config value change.

And just for reference here is the current tooltip from the 1.12.2 version:

Would make sense if the slime chunk overlay height would default to the Y 40 level, which is the topmost level in which slimes can spawn.