Roadmap for 0.92: (this bug)
=================
√ Non-pixel units are allowed but everything in a document should use the same units including guides and grids. (Guides and grids need code to do this --> see bug #1400702).
- An extension that saves a document in pixels should be added to allow backwards compatibility with 0.91.
- Optionally add viewBox if document is affected as described here:
http://wiki.inkscape.org/wiki/index.php/CSS96#Affected_files
It should be possible to avoid overwriting Inkscape version until any necessary conversion is done. If a conversion appears to be needed, a pop-up can ask the author if it should be done.
Roadmap for 0.91: (bug #1387864)
=================
√ Everything should use only pixels as is (mostly) done in 0.48 (including templates). For guides, grids, 3dbox see bug #1400702.
√ The ability to switch document units should be disabled (it's broken in several ways).
√ Font size should be stored in pixels.
See also:
=========
Related mailing list threads:
<http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/44423>
<http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/45073>
Related wiki page:
<http://wiki.inkscape.org/wiki/index.php/Units_In_Inkscape>
<http://wiki.inkscape.org/wiki/index.php/CSS96>
Original description:
=====================
Merging of experimental branch has introduced the change of px from 90 DPI as 96 DPI.
Being px the internal unit used by Inkscape to save almost everything, all measures in documents created with the 90 DPI convention are now interpreted with the new 96 DPI convention and this causes a rescaling of the entire document by 90/96 (93.75%) if compared with the physical units they were created with.
Even the page is resized, so the problem is not visually evident after opening an old document unless there is an underlying grid which has been specified with an absolute unit (e.g. mm) so objects are no more aligned to it; or until an object whose length is known is measured with a unit different than px.
This lack of evidence could cause a user to start modifying an old document and realizing there is a problem only after a while. In this situation the document has become corrupted as there are now objects rescaled and new objects with correct dimensions. Such a situation could be not easily fixable so the work done so far could be lost.
An important aspect about this issue is that many users never cared (or knew) about the 90 DPI internals of Inkscape and are probably not going to care (or to know) about the change to 96 DPI. They are only going to find their documents corrupted.
The proposed solution is to find a way to recognize old documents (differentiating the news?) and, at least, warn the user upon opening them that a rescaling has occurred. Better would be to offer an automatic way to fix this unwanted rescaling, if it's the case; in fact for some documents it would be inappropriate, for example a document whose default unit was intended to be px no matter at what DPI hence it does not have any reference to physical units inside.
Lack of doing anything about this issue could result in document corruption and data loss.
For reference, merge of experimental branch has happened in Rev. 13641.
Documents created with revisions just before 13641 are probably not affected thanks to the new viewbox approach.
Guides are always affected as they're still saved in px.
Attached document has been created with Rev. 13640. When opened with Rev. >= 13641 page, square and grid are correct, guides are not.
Zoom level and zoom correction are also affected.