Using Full Page Zoom in r31224 and running Acid2 test: After rendering the Acid2 test altering zoom (Cmd-+/-) will massively disrupt the rendering, in the same way as altering font-size with font-only scaling, whereas Page Zoom should scale the layout intact. Altering zoom before running the test results in significantly fewer layout disruptions.
Created attachment 19971 [details] Screenshot of zooming impact on Acid2 test Screenshot shows zooming Acid2 test via Full Page Zoom *after* rendering, with inset showing much less significant impact from zooming *before* rendering test.
The described problem is not a bug. As you can read a note: "When taking the test, you should use the default settings of the browser you are testing. Changing the zoom level, minimum font size, applying a fit-to-width algorithm, or making other changes may alter the rendition of the Acid2 page without this constituting a failure in compliance." [http://www.webstandards.org/action/acid2/guide/]. Similar glitches during zooming on Acid 2 test page you can observe in other browsers like Firefox and Opera.