Problem description: Users should be allowed to switch off warnings.
Steps to reproduce:
1. Use PDF/A and the following messages are shown:
During PDF export the following problems occurred:
PDF/A forbids transparency. A transparent object was painted opaque instead.
Some objects were converted to an image in order to remove transparencies, because the target PDF format does not support transparencies. Possibly better results can be achieved if you remove the transparent objects before exporting.
2. Is there a better way to display the warnings, e.g.
1. display warnings for 20 seconds and stop displaying them automatically, or
2. display warnings in an "action bar"?
In both cases, users should be allowed to do other things without confirming that they see the warnings.
At present, users must press OK or the x before they can do other things.
3. Maybe, there should be a part like:
Tools -> Options -> LibreOffice -> Warnings
( checked ) Enable warnings when XXX
( checked ) Enable warnings when YYY
( checked ) Enable warnings when ZZZ
( checked ) Enable all
( unchecked ) Disable all
( checked ) Display warnings for  seconds only
( unchecked ) Display warnings until the user clicks OK
( checked ) Display warnings on the status bar [action bar] instead
This looks systematic.
4. A possibly related issue (how users may interact with warnings when saving or exporting files) is here:
Current behavior: There is no way to stop some warnings.
Expected behavior: Users can enable and disable some warnings, and they can control how the warnings are displayed.
C. H. D.
Operating System: All
Version: 188.8.131.52 release
Confirmed under Windows XP using LO 184.108.40.206 and 220.127.116.11
Exporting to PDF/A-1a ALWAYS shows message even if it doesn't make ANY sense (e.g. when creating a PDF from a plain text document)
IMO showing a wrong error message (because it doesn't apply) is a bug.