mIRC v7.61, v7.62, v7.63, v7.64 ‒ No bug found.

Right. This means that the issue you are seeing is not the same as the one reported in this or the other thread, where it was happening with v7.62/63/64 and possibly earlier versions.

As for the reason related to the lack of available memory, there this may also be. Although I cannot say that I have a small amount of RAM. But why then, when I run older versions of the client, this bug is not there?

Because the changes to the MDI display require allocation of memory. Note that this will not only affect the MDI. Internally, it will affect many other features. The MDI just happens to be more visible.

Has the new version mIRC client with updated MDI windows become as demanding on computer resources as new computer game? ... :-]

No, it requires very few resources, but since it is GUI-related, it will be visible, compared to internal allocations for other features.

If you could not reproduce this bug

In general, when I ask users to reproduce an issue, I usually mean using a method that does not involve 1) downloading and installing a one gigabyte application, 2) that is no longer being updated, and 3) that is no longer being officially distributed :-]

Apparently something needs to be fixed, unless of course it is possibly.

As I have explained, when it comes to memory issues, all bets are off - anything can happen. This has been discussed many times in previous threads.

Thanks for trying to track it down but the issue you have found is related to memory. It is not related to the issue being reported in this thread.

However, since we are here, we might as well follow the usual steps:

1) Install mIRC into a clean, empty folder with no DLLs, scripts, etc. using the portable option in the installer.
2) Run mIRC, maximize the status window, connect to a server, join two channels.
3) Run Photoshop. Do you see the same issue?

If you follow the above steps and run other applications, other than Photoshop, can you reproduce the issue?