OK long time lurker, first post and bug report(of sorts?).
I am on latest version of mirc on Windows 10(insider program, latest build 10240). I notice now if lets say you have 3 different servers running with lets say 1 channel open on each server. If you click channel on server A, then click channel on server B then click on channel in server C. If you click server A again, it goes back to that channel, but if you click channel on server A again it goes to channel on server B even though the last channel you clicked on was channel in server C. Same thing happens if you click channel on server C again, takes you to channel on server C but if the same channel(server C) is clicked again it goes to channel on server B. I used to use this fine so I just quickly go back and forth between 2 of my favorite(main) channels. Not sure if this is a bug or not, I just know it worked before with last version on this same PC and windows version. Hope I helped out. Thanks!

I've heard others say the same, and can confirm something has changed in 7.42 there..

I'm also affected by what I think is the same bug: Since upgrading to mIRC 7.42, the "order history" of clicking channels open and then pressing escape again to go backwards in the order history is broken. This has severely impacted my workflow in mIRC as I regularly switch back and forth between channels, and now this is not possible anymore. When I now press escape it minimizes the current channel window and that grey background appears as if all windows are minimized, I always have to click the channels manually to open them again.

This issue also seems to only appear when doing this with maximized windows, non-maximized windows do not seem to suffer from this bug.

Thanks for your bug report. I was able to reproduce this issue. It was due to the mIRC v7.42 Item 27 fix in versions.txt related to this post. I have found an alternative fix for item 27 that also resolves the current issue. However, the fix still involves a change to the focusing routine, which is somewhat tricky as even small changes can have side-effects. As this bug affects the interface and will be experienced by many users, I plan to release a new version soon.

Beta has fixed this. Thanks! Will wait for full new version.