you might try jit.submatrix, so you roll your mouse over one of the thumbnails, and based upon where it is, you know which thumbnail you’re hovering over. Then use jit.submatrix to grab out that portion, run it through jit.scalebias or jit.op @op *, it can highlight by being overall brighter than the others.
Also you could have a jit.op or jit.scalebias for each thumbnail before it goes to the main matrix, if the thumbnail is selected, up the value and re-bang it into the big matrix.
jit.gl.render/jit.gl.videoplane/jit.gl.texture/jit.gl.gridshape could also do it, just have a semi-transparent gridshape over each thumbnail area, and only enable the one you’re mousing over.
Not sure about the layering in a standard jit.pwindow, would be interested to know that too.
or when you don’t need updates of the videos (for CPU saving purposes) you could use a semi transparent panel and change its position (and opacity for that matter) I tend do use panels often to do these kind of things