Wei, do you expect the draw_region passed to DoDrawQuad should clip the bottom of the perspective layer?

I captured a SKP using this hacky patch: https://chromium-review.googlesource.com/c/chromium/src/+/1264897

and then modified SkCanvas to stroke the clip path in bright green before applying it as a clip in Skia's viewer app. Image and SKP attached.

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	viewer.png 3.0 KB View Download

Deleted: pic0.skp 236 KB

pic0.skp 236 KB Download

I should note I also had this patch applied when I grabbed the SKP: https://chromium-review.googlesource.com/c/chromium/src/+/1264921

but that should just change the canvas's matrix and the path used to clip in compensatory ways, the resulting device space clip should be the same.

It's that we're not respecting the scissor rect when drawing quads. I have a potential fix for this here:

https://chromium-review.googlesource.com/c/chromium/src/+/1266655

This also fixes an issue I was seeing frequently where where web content shows up on top of chrome UI when scrolling.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8fa57c9432bf93a29c139c6c12da696171d40819

commit 8fa57c9432bf93a29c139c6c12da696171d40819
Author: Brian Salomon <bsalomon@google.com>
Date: Tue Oct 09 17:22:09 2018

[SkiaRenderer] Implement scissor rect for DoDrawQuad

Bug:  892764 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I90eb544ba6ea5c8127374b2ba65f80d2b6cbc075
Reviewed-on: https://chromium-review.googlesource.com/c/1266655
Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Cr-Commit-Position: refs/heads/master@{#597969}
[modify] https://crrev.com/8fa57c9432bf93a29c139c6c12da696171d40819/components/viz/service/display/skia_renderer.cc