i don't know if this is right or not...is a nice effect (i don't have anywhere seen a composite on composite on composite effect like that .. in other video-editors i mean)
the secondary composite are rescaling primary composite... (very hard to wirite in C++ code a thing like that)

but.. usually video editors are just joining 2 o 3 videos at the same time (or 4 for a security camera split view).. they can move every video on the correct place, with correct size very easily.

i think that this "matrioska effect" is a bug .. it can be avoided just making reference to "black" track on composite parameters...
probably fixing the mouse red area on this sistuations is very hard for kdenlive developers.

and probably this strange behaviour is also the BUG that make "trasparencies" wrong on texts or images when using multiple "composite".
see this bug: https://bugs.kde.org/show_bug.cgi?id=384581

and an image: https://bugs.kde.org/attachment.cgi?id=107792