Can you please provide a self-contained test case?

I added a new test in https://github.com/KhronosGroup/WebGL/pull/2523 which indicates that there's no bug in Chromium. More likely there's a bug in the application.

https://klacansky.com/open-scivis-datasets/

Open first dataset (Aneurism) by expanding the arrow. Then in preview when you rotate with mouse it works as expected (uses extra gl.clear) but zoom by scrolling does nothing (I commented out the gl.clear). If you rotate after zooming you will see the update.

Thank you for providing more feedback. Adding requester "kbr@chromium.org" to the cc list and removing "Needs-Feedback" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Thanks, can reproduce the behavior. A smaller self-contained test case would have been helpful, but have yours running locally. Investigating.

Thanks again for the test case. https://chromium-review.googlesource.com/706602 up for review fixing it and adding a regression test. Unfortunately since the WebGL conformance suite doesn't yet support reftests this can't be tested there since the bug here was in the compositing of the WebGL canvas; the bug can't be seen from JavaScript.

Also: https://github.com/KhronosGroup/WebGL/issues/2527 filed about being able to add tests for bugs like this to the WebGL conformance suite. (The bug here could only be seen visually, and not from JavaScript.)

Thanks for quick fix and reporting the conformance tests issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0916d55b226948b79bb46ba63b8649e3448f0d18

commit 0916d55b226948b79bb46ba63b8649e3448f0d18
Author: Kenneth Russell <kbr@chromium.org>
Date: Mon Oct 09 19:51:10 2017

Fix BlitFramebuffer's context dirtying.

Mark the context changed if the destination of the blit is the default
back buffer. Added new pixel test.

BUG= 768969 

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I30d8c6df99c3dc32466d55157c9e9d31acf8ef38
Reviewed-on: https://chromium-review.googlesource.com/706602
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507454}
[add] https://crrev.com/0916d55b226948b79bb46ba63b8649e3448f0d18/content/test/data/gpu/pixel_webgl2_blitframebuffer_result_displayed.html
[modify] https://crrev.com/0916d55b226948b79bb46ba63b8649e3448f0d18/content/test/gpu/gpu_tests/pixel_test_pages.py
[modify] https://crrev.com/0916d55b226948b79bb46ba63b8649e3448f0d18/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp

After writing the above CL I realized the same bug exists in the new WebGL 2.0 entry point clearBufferfv.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d47ea05b31b129fa6bec5393858cccd85d66cf41

commit d47ea05b31b129fa6bec5393858cccd85d66cf41
Author: Kenneth Russell <kbr@chromium.org>
Date: Fri Oct 13 22:28:27 2017

Properly mark canvas changed in clearBufferfv.

Simplify earlier fix to blitFramebuffer; forgot that MarkContextChanged
already checks if the draw framebuffer is the default back buffer.

BUG= 768969 

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ifdb21a2689fd3d9a9e694863df50bbb9e8588521
Reviewed-on: https://chromium-review.googlesource.com/714653
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508849}
[add] https://crrev.com/d47ea05b31b129fa6bec5393858cccd85d66cf41/content/test/data/gpu/pixel_webgl2_clearbufferfv_result_displayed.html
[modify] https://crrev.com/d47ea05b31b129fa6bec5393858cccd85d66cf41/content/test/gpu/gpu_tests/pixel_test_pages.py
[modify] https://crrev.com/d47ea05b31b129fa6bec5393858cccd85d66cf41/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/09a81d3609c63f62d4775e087cbefdd7468da170

commit 09a81d3609c63f62d4775e087cbefdd7468da170
Author: Kenneth Russell <kbr@chromium.org>
Date: Tue Nov 14 06:54:56 2017

Roll WebGL 34842fa..12192b9

https://chromium.googlesource.com/external/khronosgroup/webgl.git/+log/34842fa..12192b9

BUG= 765469 ,  768969 ,  769989 , 772651

TEST=bots

TBR=zmo@chromium.org, kainino@chromium.org

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I180314943cb6427b916790f5ae5bf295c87330ea
Reviewed-on: https://chromium-review.googlesource.com/764818
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516218}
[modify] https://crrev.com/09a81d3609c63f62d4775e087cbefdd7468da170/DEPS
[modify] https://crrev.com/09a81d3609c63f62d4775e087cbefdd7468da170/content/test/gpu/gpu_tests/webgl2_conformance_expectations.py
[modify] https://crrev.com/09a81d3609c63f62d4775e087cbefdd7468da170/content/test/gpu/gpu_tests/webgl_conformance_expectations.py
[modify] https://crrev.com/09a81d3609c63f62d4775e087cbefdd7468da170/content/test/gpu/gpu_tests/webgl_conformance_revision.txt