Thanks for filing this, some notes.  Perhaps use the APNG we added on 

https://crrev.com/427dfb952160b6402103f5db70a0e3dc6c5c500d/third_party/WebKit/LayoutTests/images/resources/count-down-color-test.gif

Copy http/tests/images/jpeg-partial-load.html to http/tests/images/png-animated-partial-load.html, and use the image above in it, set the mimeType for animated PNG (image/apng, I believe), then manually work out the &stallAt= value.

That value is the offset into the encode file stream bytes to stall at.  Set it so that the test draws ~1/2 the initial decoded image frame (frame 0).  This will require some fiddling about to determine the value in bytes.

Indirectly, this test will also check the decoder's alpha handling of the first frame.  The background CSS color should be seen (green IIRC) since the frame is partially decoded (it's state is not ImageFrame::FrameComplete), it should have alpha everywhere outside the decoded regions.

HTH.

>https://crrev.com/427dfb952160b6402103f5db70a0e3dc6c5c500d/third_party/WebKit/LayoutTests/images/resources/count-down-color-test.gif

Ahem, that's a GIF, meant the APNG image of course ...

https://chromium.googlesource.com/chromium/src/+/427dfb952160b6402103f5db70a0e3dc6c5c500d/third_party/WebKit/LayoutTests/images/resources/count-down-color-test.png

https://codereview.chromium.org/2743253009/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2ef26ee13e1d61e0d52daf64bd4ad2ef7a7907d5

commit 2ef26ee13e1d61e0d52daf64bd4ad2ef7a7907d5
Author: noel <noel@chromium.org>
Date: Thu Mar 16 21:44:07 2017

Add a partial load test for an animated PNG image

Use HTTP test load-and-stall script to partially load the initial
frame of a animated PNG image.

Render the image over CSS background color green and confirm that
the CSS color appears in the undecoded regions of the image.

BUG= 700892 

Review-Url: https://codereview.chromium.org/2743253009
Cr-Commit-Position: refs/heads/master@{#457569}

[modify] https://crrev.com/2ef26ee13e1d61e0d52daf64bd4ad2ef7a7907d5/third_party/WebKit/LayoutTests/TestExpectations
[add] https://crrev.com/2ef26ee13e1d61e0d52daf64bd4ad2ef7a7907d5/third_party/WebKit/LayoutTests/http/tests/images/png-animated-partial-load.html

Results look good, watching it run around the bots for some number of cycles to check results are stable (not flakey).

> Ahem, that's a GIF, meant the APNG image of course ...

And we should add a test for an animated GIF too, since APNG and GIF test coverage came up while reviewing https://codereview.chromium.org/2749703002  bug 701143 .

Animated GIF case done on  bug 702148 .

#7 Stability looks good.

Deleted: png-animated-partial-load.png 331 KB

	png-animated-partial-load.png 331 KB View Download

Result looks good, CSS green background color visible in undecoded parts of the image.

Deleted: png-animated-partial-load-result.png 23.1 KB

	png-animated-partial-load-result.png 23.1 KB View Download

This to cover any changes in https://codereview.chromium.org/2749703002 and the patch split out from there.