Finally!!!!
An answer that makes sense.
It may be that use of complex custom dialogs may also exhibit the same memory leakage, due to the way mirc creates/destroys windows.
I just looked at my running mirc (6.03) and it has 336 "USER Objects" in Task Manager (Windows 2000 SP4).
I right click in a channel to get the popup menu, and it jumps to 407 objects, clicking off the menu, it drops back down to 336.
Selecting one of the items to open my "complex dialog", it jumps to 574 USER Objects.
Now I look at ram usage....
I minimize mirc, then restore it, then wait for it to settle out.
(changes by less than 15K/sec, due to being on multiple networks and multiple channels)......
Right click in channel to get menu, Ram usage goes up 1 meg.
Right click off to make the menu go away..... Ram Usage does not recover.
Right click on channel to get menu, negligible change in Ram, select item to open my dialog.... Ram Usage goes up 4 megs
Close dialog, ram usage does not recover, even though the USER Objects returns to 336 as before.
Subsequent opening of my dialog cause Ram usage to go up 200K each time, which does not recover until I minimize mirc.
Using other software (TaskInfo 2003), I see that "In Ram KB" is reduced when you minimize mirc, but "Private KB" does not change one bit.
Bottom line, something in mirc causes it to use more ram when you open menus and dialogs, and this ram is not being released when those items are closed.
Good job on your explanation.
