I have a PDF including bookmarks automatically generated from DocBook
part and article titles.  One part has an empty title, and although a
bookmark is created for it, neither evince nor okular display that part
or any of its subparts in the bookmark listing.  Obviously, this makes
navigation more difficult.

To illustrate the matter clearly, I've included the DocBook 5 source as
well as the PDF in question.  If you look at the contents of the PDF
(say, in less), a bookmark is generated for each DocBook part and
article, including the one with the empty title.

I think that poppler and the programs that use it should display the
full bookmark tree, even if the title of one of the parts is empty.

Please let me know if you need more information or if you need me to
test something.

I've looked at the source and I see that in poppler's
OutlineItem::OutlineItem, titleLen is set to 0 if the item's Title key
is not a string.  This is a problem since 0 is a valid length, but it's
being used as an indication that the OutlineItem is not valid.

I'm not sure that there's an easy way to fix this, but it should
definitely be forwarded upstream so that it can be fixed in a consistent
way that will work in all the poppler-using programs.

tags 639614 + patch
kthxbye

To fix this bug, I first tried to make sure that the title member of
OutlineItem was non-NULL if titleLen is 0.  This, unfortunately, does
not fix the problem.  Neither okular nor evince render the missing
outline trees.  It's unclear to me what is intrinsically preventing them
from rendering outline items with a zero-length title, since this is
perfectly valid according to PDF 1.4.

As a workaround, I've changed the code so that if the length of the
string is 0, to create a string containing only the Unicode character
U+200B, ZERO WIDTH SPACE.  This works in both programs, but it's really
just a hack.  The code should be fixed to do something sane and
reasonable, but I'm not familiar enough with it to determine what that
should be (although it very well may break the ABI, which is even more
reason for a deliberate and well-thought-out decision).

Control: forwarded -1 https://gitlab.freedesktop.org/poppler/poppler/-/merge_requests/1427
Control: tags -1 +patch

A patch submitted upstream.