If you use Command-Plus or Command-Minus you can never get back to 100% scale unless you close and reopen the Inspector. This happens because we scale by multiplying the current zoom by a percentage. That is not undoable by doing the reverse. Instead we should add or subtract from the current zoom level.
<rdar://problem/23303488>
Created attachment 264252 [details] Patch
Comment on attachment 264252 [details] Patch r=me
Comment on attachment 264252 [details] Patch Clearing flags on attachment: 264252 Committed r191697: <http://trac.webkit.org/changeset/191697>
All reviewed patches have been landed. Closing bug.