when you left click the window (where the writing comes up) and its maximised, u can see some flickering upon redraw
usually the flickering occurs on the top half of the window

im not sure if its just me, but i tested it on a fresh mirc

Confirmed (on 6.16).

Confirmed here but no clue how this is a "Bug"

I can't seem to reproduce here. Maybe I'm doing something
wrong. Does it happen in any window, channels, queries?
Maybe it depends on your video refresh rate or your
resolution setting?

v6.16
XP Pro SP2
1024x768
85Hz

hmm im on SP1 only
same reso, 75mhz refresh

well its not really a 'bug', but its not exactly pleasant when you have a screen full of text i guess

just reporting my findings