I have 15 server tabs open right now. I server -m'd the 15th and it opened in 14th spot. Now when trying to click on one they will rearrange and minimize each other. I don't have anything new that should cause this behavior and the only thing I have that would remotely seem like it could be involved is a dynamic highlight window that associates with the active connection. (It's closed atm) I don't know what else to explain but if I can answer any questions I'd be glad to. This is v7.5 on Win 7 Ent.
I just noticed that they rearrange when clicking on other server tabs as well as just the 14th and 15th. Sometimes they switch spots two or three times when I click on another network.
If you test this in a clean install of mIRC, with no scripts or addons, does it still happen? Can you provide a step by step example, or a simple script, that I can use to reproduce the issue in a clean install?