From our QE:
Let me try clarifying things a bit more:
The Undo Stack fix that's in HotFix6 is really about minimizing Undo Stack flushes that get triggered by the garbage collector. This was because the data we got from the users who used our special build with the intstrumentation tool, pointed us in that direction, meaning a vast majority of the flushes they were reporting seemed to be related to that.
Time-wise or operation-wise, these flushes happen in a non-deterministic way, thus the random aspect of stack flushing in these cases. This HotFix was really aiming at fixing this sort of issue, but was not claiming we would resolve all Undo Stack flushing issues. Some - like the Material Editor one - got fixed in 3ds Max 2010 through other code changes, like what was done for improving the Reference system. Others are still present, like the case of the object deletion while in object creation mode.
It's also for being able to get more data on other problematic situations that we decided we would also include the diagnosis tool in the HotFix (i.e. the instrumentation system), so that users would have a way to send data about specific Undo stack flushing issues they might encounter. This diagnosis tool can be enabled by setting an environment variable, and we might want to ask users, who would still report Undo stack flushing problems, to turn it on and send us CER reports, for helping us figuring out what would still trigger unwanted flushes.
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So, if you're having undo problems after installing the hotfix, we probably need to turn on the special "instrumentation system" to get more data on your problem.
