In Version 6.61b1 build 4195:
After switching between upper and lower window, the upper window loses focus.
Custom toolbar buttons added manually became inactive.
Highlighted posts in the upper window looks like unmarked.
Only right click on the highlighted posts returns focus to the upper window and makes the custom buttons active.
In 6.60 all was OK,
To move the focus to the upper window was enough to click on the header of the upper window,
buttons also becomes available.