Visual artifacts for DXT-5 textures (black-blocks) in PepperFlash plugin

Problem Description:

Sometimes ATF DXT-5 textures decoded with visual artifacts: black blocks

Large textures corrupt more frequently than small ones

That artifact shows up only in PepperFlash plugin (Chromium-based browsers)

Steps to Reproduce:

• Goto https://rawgit.com/mrchnk/FlashPlayerDXT5Bug/master/bin/index.html
• Wait until frame counter stops with something visual like https://i.imgur.com/tntw9JV.png

You can grab full source code and assets from https://github.com/mrchnk/FlashPlayerDXT5Bug

Actual Result: Texture rendered with artifacts

Expected Result: Texture rendered without artifacts

Any Workarounds: Re-upload texture.

It's hard to automatically detect is it decoded normally or not.

Issue tracker link: https://tracker.adobe.com/#/view/FP-4198484

This bug still shows up in chrome version 66 and plugin version 29

OS version: Windows 10 x64, MacOS

GPU: various NVIDIA's and AMD's GPU's including gtx960, gtx970, gtx1070, gtx1080

I would like to start the discussion about that bug and found all affected machine setups.