Some additional information...
One client having the problem had the Super Container Companion Plugin installed but it was v2.77. However, updating this to the 2.852 version did not fix the problem.
We found that she could drag and drop files onto the SuperContainer web viewer and the files uploaded as expected but it is when the choose file... button is clicked on that the flashing occurs. Although this was reported as an intermittent problem, we could reproduce it every time we tried today.
The specs on the OS and related applications are...
Windows 7 Professional SP1
FileMaker Pro 11.0 v3
SuperContainerPlugin 2.852
Java Standard Edition v6 update 29 build 1.6.0_29-v11
Hope that helps track it down.
Regards
Andrew
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Andrew McCallum
Niche IT Pty Ltd
Sydney, Australia