Strange freeze at "Move to New Window"

[Eugene Loza]

When working with several Source editor, I get a strange freeze when trying to "Move to Other Window > New Window" (see screenshot). Lazarus freezes for indefinite amount of time with a pop-up window, however, after a few dozens of seconds (not instantly) pressing a cross on the pop-up windows resumes normal operation. The window doesn't unfreeze by itself.
I couldn't reproduce the issue on a "clean" project.
"Clone to Other Window" works flawlessly.
I'm not sure whether it relates to the issue, but once a window with a package (Cody or CodeTools? I didn't pay attention then) popped up.

Lazarus 1.8.0RC3, FPC3.0.3, Debian Jessie 64 bit.