You need to deactivate this functionality in X11 preferences > Pasteboard: uncheck “Update Pasteboard when CLIPBOARD changes”. It currently does not know how to deal with vector images, so it just captures the screen, i.e., creates a bitmap copy, and then pastes that. Starting with XQuartz 2.3.2, X11 has some functionality to exchange the content of the clipboard with OS X.
Sure enough, a quick search took me to the inkscape FAQ, and this section in particular: After beating my head against a wall trying to create a pattern-on-path effect, I realized that there must be something wrong with the clipboard implementation on the mac. I’ve always just resigned to use clone (ctrl-D) instead of copy/paste since on the mac, the copy/paste cycle results in a pixelated image being pasted into the document.
Inkscape is my go-to vector editing program and I’ve done many publication figures, exam questions, and other general work in inkscape on both mac and linux.
