Jit.gl.videoplane layers using transparent alpha

Hello,

I'm trying to have my patch render two side by side videos which are rendered over a white layer. The idea is that when the video matrices are in their normal position they cover the white layer. But sometimes we move them vertically - then you should see white above or below them.

I figured I could do this with alpha-blending in opengl, and I even thought I found the answer on this forum, but it's not working consistently for me. Although it seems to always work in a floating window, when I go to fullscreen the results are inconsistent. Sometimes it works; other times the whole frame renders white.
I've played around with different depth_enable and blend_enable settings, but it doesn't seem to work. I haven't worked all that much with the jit.gl stuff, so it's likely I'm missing something basic or documented elsewhere, but I can't seem to find it. Render portion of patch is attached - thanks for any tips!