bug: Hide shader controls hides invlerps and breaks interaction with transfer functions

See [here](https://neuroglancer-demo.appspot.com/#!%7B%22dimensions%22:%7B%22x%22:%5B8e-9%2C%22m%22%5D%2C%22y%22:%5B8e-9%2C%22m%22%5D%2C%22z%22:%5B8e-9%2C%22m%22%5D%7D%2C%22position%22:%5B17213.5%2C19862.5%2C20697.5%5D%2C%22crossSectionScale%22:1%2C%22projectionScale%22:65536%2C%22layers%22:%5B%7B%22type%22:%22image%22%2C%22source%22:%22gs://neuroglancer-janelia-flyem-hemibrain/emdata/clahe_yz/jpeg/%7Cneuroglancer-precomputed:%22%2C%22tab%22:%22rendering%22%2C%22opacity%22:1%2C%22blend%22:%22additive%22%2C%22shader%22:%22#uicontrol%20invlerp%20contrast%5Cn#uicontrol%20transferFunction%20tf%5Cn#uicontrol%20vec3%20color%20color%5Cnvoid%20main%28%29%20%7B%5Cn%20%20float%20contrast_value%20=%20contrast%28%29%3B%5Cn%20%20if%20%28VOLUME_RENDERING%29%20%7B%5Cn%20%20%20%20emitRGBA%28tf%28%29%29%3B%5Cn%20%20%7D%5Cn%20%20else%20%7B%5Cn%20%20%20%20emitRGB%28color%20%2A%20contrast_value%29%3B%5Cn%20%20%7D%5Cn%7D%5Cn%22%2C%22shaderControls%22:%7B%22contrast%22:%7B%22range%22:%5B94%2C187%5D%2C%22window%22:%5B71%2C207%5D%7D%2C%22tf%22:%7B%22controlPoints%22:%5B%5B0%2C%22#000000%22%2C0%5D%2C%5B62%2C%22#ffffff%22%2C0.4549019607843137%5D%2C%5B205%2C%22#ffffff%22%2C0%5D%2C%5B255%2C%22#ffffff%22%2C1%5D%5D%7D%7D%2C%22volumeRendering%22:%22on%22%2C%22volumeRenderingDepthSamples%22:256%2C%22name%22:%22jpeg%22%7D%5D%2C%22selectedLayer%22:%7B%22size%22:479%2C%22visible%22:true%2C%22layer%22:%22jpeg%22%7D%2C%22layout%22:%224panel-alt%22%2C%22helpPanel%22:%7B%22row%22:2%7D%2C%22settingsPanel%22:%7B%22row%22:3%7D%2C%22toolPalettes%22:%7B%22Shader%20controls%22:%7B%22side%22:%22left%22%2C%22row%22:1%2C%22query%22:%22type:shaderControl%22%7D%7D%7D) for an example.

I think the transfer function issue is that for some reason it is constantly recreating the element containing the shader controls.

Start with the shader controls and transfer-function behavior shown in the linked Neuroglancer reproduction. Trace how hiding shader controls affects invlerp controls and how the transfer-function element is recreated. Done means hiding the controls no longer hides invlerps and transfer functions remain interactive.

Written by the indexing model from the issue text.