When a block is inserted from a linked file, selecting any object in the file afterwards causes the object properties sidebar to grow. Deselecting shrinks it again. The amount it grows seems to depend on the length of the filename.
To reproduce,
make a simple file and save with a long ( 50 character) filename.
open another file, insert the previous file as linked only block.
Now any object selected causes the sidebar to grow.
It does NOT happen if:
the file is inserted as an embedded or embedded and linked file.
the “name” field on the insert dialog is reduced from the original filename
the object properties are not visible in the sidebar.
Using rhino 5.3.2 on OSX 10.10.5