Overview:
On middle click yakuke pastes content of clipboard instead of primary selection.

Steps to Reproduce:
Copy some text.
Select some text (I tried selecting text in yakuake and in other apps).
Click middle mouse button in yakuake. (previously copied text will be pasted)

Actual Results:
On middle click pastes content of clipboard.

Expected Results:
On middle click pastes content of primary selection.

Build Date & Platform:
Linux 4.15.15-1-ARCH #1 SMP PREEMPT Sat Mar 31 23:59:25 UTC 2018 GNU/Linux

Additional Information:
On my configuration it works as expected in konsole and firefox.