cheers. I've used the instructions from:https://geekwentfreak-raviteja.rhcloud.com/blog/2012/10/10/modify-dependencies-addremove-files-of-debian-deb-package/
to remove libunity9 as a Pidgin requirement, and installed the modified deb.
Pidgin seems to behaving normally.
I suspect the libunity9 requirement has been added in error. I've logged a bug against pidgin on launchpad (1605106) to get clarification as to whether libunity9 should really be a requirement, given that pidgin seems to operate perfectly well without it.
With libunity9 gone, window control buttons are restored in Chrome and Chromium.