Whatsapp Web spans primary selection buffer in Chrome and Firefox on Ubuntu 14.04

Every time I type a key in Whatsapp Web 1 or 2 of the previous keys I typed are put into primary selection buffer (mouse middle-button click).

This only happens when typing within the new message composition field. Deleting or pasting text does not affect the primary selection.

Tested on Ubuntu 14.04 with both Google-Chrome and Firefox, accessing the link http://ift.tt/1Dt3Tno.

Can anybody confirm this?