I am running in Windows 98. I just finished recreating my whole workspace from a clean new copy of the "8 Standard" Workspace, and after a bit of weirdness, it all seems to work as I want. However, the first time I tried to edit a copy of "8 Standard" I had the same problems with "M" and Unconstrained Linear Dimension, then I tried a few more times and eventually it worked... perhaps this is a version of the "turn it off, wait 30 seconds, turn it back on again" school of computer repair... Thanks for your help - it seems to be ok now - I hope it stays that way.
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