There is a different behavior with the focus for the editbox in 6.3 and in 6.21, here is the step to reproduce :
-you should have two channel window open
-The focus is on the editbox of the active channel
-Select a nick in the nicklist
-The editbox lost the focus
-Click on another channel
-Comeback in the previous channel
In mIRC 6.21, the editbox of the first channel will has the focus, on mIRC 6.3, it's not the case
The behavior in the mIRC 6.21 was much better