If "Export" would behave like "Save copy as", the changes wouldn't be lost.

I just realized that perhaps that was a little unclear...

I know that the dialog says 'exporting won't change the original', but the
original window goes away. I would have to 'save as' xcf again. I guess what
would be better is that the original window is still open and the new image gets
it's own.

That's exactly what "Save a Copy" is for.

Sven, I understand what save copy as is. That is not the point. Due to an
unclear interface, I lost data. When I closed the exported copy, I thought my
XCF would still be around.

Errr, well, you misinterpreted the user interface then. Saving a document means
to change it's name. So if you save as foo.png, you change the name of the image
to foo.png. Perhaps the term Export is misleading here? The export step is
something the plug-in does to help you to save to the format you've choosen.

Perhaps I haven't been clear here. Remember that you did not Export your image.
What you've choosen from the menu was "Save". The fact that the image needed to
be converted to fit the chosen file format does IMO not warrant to change the
semantics of Save. For exactly the use case you've outlined here, we've added
"Save a Copy".

With all due respect, it seems that you are unconcerned that your (especially
the ignorant ones) Can lose data.

You say that I didn't export, but I did! The save dialog took me there changing
my intended task to export, because the software was smart enough to realize
that XCF and PNG are incompatible save targets. Perhaps this is an unexpected
path for a user to take, but  *it is possible to lose data*. The export dialog
told me "export conversion will not modify the original", very true. But it
certainly modified my working copy.

I don't expect to convince you that export and save as are the same thing, but I
would argue that novice users can't see the difference (e.g. ME :-)

May I kindly suggest that it work like this:

1: User opens XCF
2: Changes made
3: User clicks Save ("the wrong button")
4: Export says "I won't modify original, but I am gonna toss the changes you
made. Would you rather save as?"

If step two NOT taken, I wholeheartedly agree that the current functioning is
correct.

Export doesn't modify the original nor does it toss away any changes. The only
modification to the copy that happened was that you've choosen to Save under a
certain name. Of course the image's name is changed then. This works this way in
every software I've ever seen. The only thing you could argue is wrong here is
that saving to PNG does unset the dirty flag even though the PNG format is not
necessarily capable of storing all image information. Changing that would be a
major inconvenience though.

Hmm. When thinking about it it seems like a good idea to keep the image dirty
when the export dialog was necessary. This would not cause the inconvenience for
images that *are* suitable for e.g. the PNG format and it would not cause the
data loss as described in the original bug report.

We would have to keep the image dirty for all formats except XCF then since no
other image format is capable of handling all information in a GIMP image.

We right now have a solution that encourages data loss as described in the bug
report. My suggestion is a compromise between convenience and perfectionism. I
don't think that the loss of meta-data, paths etc. is as dramatic as the loss of
a complex layer structure, hence the export dialog is a good (and user visible)
place to draw the line.

I don't think the current UI encourages data loss and I don't think the
suggested solution is reasonable. What I could imagine is to change Save to
always save to the XCF file format and add an Export menu item to export the
image to other file formats. The export filename would then have to be attached
to the image so that exporting again doesn't necessarily open the file chooser.

Hmm...I think that what Sven proposes in #12 is too radical - it open space 
for people to criticize GIMP as not been able to save in different formats. 
(Belive me the world is full of such people when one tries to advocate Free 
Software). 
What Simon puts on #9, however seems reasonable. It is what OO.org already 
does, and works fine. Just keep the the image flag dirt, and find a way to 
show up a  different "unsaved warning", when closing/quiting than the one 
displayed for a non saved image at all.