Is it possible to add a search text editor widget to the 3rd tab to help find the item of an entity prototype? And if it's possible not to display all the nil variables?
Or should I simply do it in the first tab with game.entity_prototype["the-entity-here"]? Or assign it to some global variable in order to watch it in the second tab?

For now, viewer doesn't have search function. I think it is quite hard work for me now, because it has tree gui. I have been thinking how to make it with factorio gui system.
I recommend to make a new global variable tracking sub property of an entity.

I tried 1 or 2 way but failed. I can't remember the detail. The simplest way might be, a check box to control whether to display the nil attributes. The same way as whether to display the function members. I went over some of the vanilla lua code to see how they implemented the prototypes. I found the index strings for different prototypes are different. Some are very short with very few attributes.
I don't know. If this is not possible, it could be harder than expected.

I saw the update. Good job!