With --site-per-process:
(1) Open a new about:blank tab. 
(2) From devtools, insert a cross-site iframe with a CSS transform to rotate it 45 degrees:
    
  var f = document.createElement("iframe");
  f.src = "https://csreis.github.io/tests/input-types.html";
  f.style = "transform: rotate(45deg)";
  document.body.appendChild(f);

See screenshots for what happens.  Correct behavior (without --site-per-process) is for the frame to appear as a rectangle rotated 45 degrees.  With --site-per-process, we get a weird pentagon-like shape.  If you scroll around in the frame, it's also using the incorrect shape rather than the rectangle. 

Note that if you apply the transform after inserting the frame (i.e., reverse the order of last two JS lines), things work correctly.  So this is something specific to the initial path.
 

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	expected.png 91.2 KB View Download

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	actual.png 118 KB View Download