+Chris,

I notice that one of your tests also shows a similar behavior, not sure whether you know something about it?
https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/paint/paint_layer_clipper_test.cc?type=cs&q=%23if%5Csdefined%5C(OS_MACOSX%5C)+f:test%5C.cc+f:paint&g=0&l=301

The code I wrote that you linked to is about the control clip of a native-rendered select element.

But are you testing such a situation? I think the link you gave may be out of date. What case are you
testing?

I also use #ifdef here.
https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/paint/image_paint_timing_detector_test.cc?q=image_paint_timing+package:%5Echromium$&dr=C&l=304

I was testing the image size. When we set an image of a certain size, the layout size will turn out to be one pixel larger in width, in MacOs only. This looks like the same issue with yours.

chrishtr@, I'm marking this available, let me know if you disagree.

The issue is due to font heights of inlines being platform-dependent.

<img> is by default display:inline, which causes it to use vertical positioning
according to the "font" that is used by its inline range.

You can fix it easily by changing the images to display:block in your unittest.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2e7cb796ead3315ea3795ac7813e89da23e0d27a

commit 2e7cb796ead3315ea3795ac7813e89da23e0d27a
Author: Liquan(Max) Gu <maxlg@chromium.org>
Date: Mon Jan 07 20:12:24 2019

[FCP++] Set Display:block to image paint unit tests

As the font heights of inlines is platform-dependent, the height of image in
Mac and other platform are determined differently.

<img> is by default display:inline, which causes it to use vertical positioning
according to the "font" that is used by its inline range. As a result, the
unittest shows different height for the image on different platform.

To fix it, I change the images to display:block in the unittest.

Bug:  894234 
Change-Id: I43a582ee41e1bacb3025e2487135e2cbf0a69887
Reviewed-on: https://chromium-review.googlesource.com/c/1398520
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620443}
[modify] https://crrev.com/2e7cb796ead3315ea3795ac7813e89da23e0d27a/third_party/blink/renderer/core/paint/image_paint_timing_detector_test.cc

Thanks Chris, the problem has been fixed:)