Disappearing edit commands

[graham samuel]

I have a longish script in a stack (about 800 lines: not **that** 
long), and I find that while I'm working on it using a Mac running 
10.3.8 and RR 2.5.1, my ability to do copy the script's text with 
Command-C, cut with Command-X and paste with Command-V just disappears. 
I get a flash in the IDE's menu bar, but no result. If I choose these 
functions from the IDE's menu, everything works as normal. Other 
scripts in the same file don't seem to suffer from this problem, and I 
don't appear to have any other odd side-effects.

Has anyone seen anything like this? I can't guess what I'm doing wrong.

Graham
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Graham Samuel / The Living Fossil Co. / UK and France