Plug-in UI elements have disappeared and reinstalling does not fix problem

[moosehill]

New user here.

I am using QGIS 3.34.12-Prizren with v2.8 Qgis2threejs to visualize DEMs.

Everything was working -- I loaded a DEM, started the plugin, selected the correct layer and had rotatable 3D renderings. (Very nice!)

As a new user, at some point I clicked each of the two tiny icons to the right of the word "layers" (to learn what they did) and either immediately or soon after the entire lefthand Layers element disappeared. I have not been able to restore it. The plugin was still correctly rendering DEMs, but at some point that also stopped working as well and I no longer saw the "working" indicator on the bottom of the plugin screen. Now the plugin window is blank and nothing is rendered. I have tried repeatedly to fix this.

Realizing I must have done something wrong, I uninstalled the plugin, shut down QGIS, restarted QGIS, and then reinstalled the plugin.

Now when I try using the plugin, the screen is blank again (see attachment). No Layers UI elements, nothing. And no renders. (If this is useful, I see that the axxis display still rotates when I click and drag in the window.)

My real question is: How do I -completely- reset everything so that it works again? I would think that uninstalling should remove all previous settings, but that does not seem to be true. Something remains broken across multiple attempts to delete and reinstall the plugin.