I was just playing around with a 3d box. I'm able to edit the box at will, until I change Doc Prop > Page Orientation from the default Portrait to Landscape. At the moment Landscape is clicked, the page border changes (as expected) and the 3d box jumps to this point -- X: 453.00, Y: 699.421. When I try to move the box, it snaps back to that exact point, every time. If I switch back to Portait, the problem persists. The box always snaps to that exact point, every time.
Now I've just opened a fresh doc, created a 3d box with different dimensions. Again, I can move it around anywhere, successfully, as long as I remain in the default Portrait orientation. But if I switch to Landscape, the box jumps to -- X: 171.00, Y: 645.974. Again, it snaps back to that point exactly, every time I try to move it. And again, switching back to Portrait does not resolve the problem. It still snaps to that exact place every time I try to move it.
Now another fresh new doc. This time, changing to Landscape orientation before drawing the box. No problems, box moves without snapping away. Switch to Portait, no problems, box moves anywhere without snapping away. So this is clearly a good workaround, if the problem has not already been fixed in the development version.
I'm using Inkscape v 0.47, last stable release on Windows 7 laptop (64-bit). I searched Launchpad and found some similar issues, but none exactly like this. Here are the similar ones (or at least they appear similar in certain ways, to me):
https://bugs.launchpad.net/inkscape/+bug/187668
https://bugs.launchpad.net/inkscape/+bug/400982
There may be others that I didn't find because I only searched with the Launchpad engine, not Google.
So is this a bug? Should I report it? Or has it already been fixed in development version? OR am I missing something about using Inkscape, somewhere. Here, let me upload the SVGs for you.
Ok, new problem, possibly related. After I saved those 3 files, and before uploading, I closed 2 and 3 (leaving the original open to continue working). Then I had an afterthought and wanted to check on something, so I opened them again. On the file containing the 3d box that can't be permanently moved (#2), the box is invisible! In the Open dialog, in the list of files that I might open, the boxes are visible, but after I click on the file to choose it, the Preview shows the box invisible (or iow the box doesn't show up). And after I open it, the boxes are invisible. I have not used layers, or even applied any color, much less opacity. These are very basic files. If I click Edit menu > Select All, the info area says "3D box in layer Layer 1" but I don't see it, not even the selection box. I opened the Fill and Stroke dialog to see if somehow the opacity had been changed, but it shows 100% opacity! Layer also 100%!
But on the 3rd file, containing the box that can be moved around without being snapped away, the box is visible. So I'm not sure how you can actually see the boxes, but the files are there for you

Thanks for any help you might offer. For now, I can just use the workaround of changing the orientation before I draw the box. So not a huge problem. But still possible a bug (or 2). Thanks again

Edit: Went back to the original file, and thought I would copy the 3d box into a new doc with orientation already set to Landscape, rather than draw it again. Copy and paste. Even though the box is still visible in the original file (that was never closed), the pasted box is invisible, not even the selection box can be seen, but again, the info area says "3D box in layer Layer 1". How weird is that? Oh well, I guess I'll just draw the box again then
