Alpha blend rigged mesh turns invisible when "Highlight Transparent" is enabled

[canny]

Steps To Reproduce

• This bug only affects some rigged mesh with alpha blended textures. I have so far been unable to create a simple repro object from scratch which will reproduce this bug

• Repro item 1: Clover Megalashes: https://marketplace.secondlife.com/p/Clover-MegaLash-DEMO-add-me/24449814

• Repro item 2: Nova Alloy hair: http://maps.secondlife.com/secondlife/Nova/110/181/428 - this one - https://prnt.sc/3S8bxQcMNFoI - the demo is fine to repro.

• Attach the Alloy hair and the Clover Megalashes to your avatar

• Enable "Highlight Transparent via CTRL+ALT+T or Top Menu -> Build -> Options -> Highlight Transparent

• Observe how the hair and lashes render.

Observed Behaviour

• When Highlight Transparent is enabled, most of the Alloy hair turns invisible and part of the Clover megalashes turn invisible

• See attached image

• Bug also reproduces onn Firestorm Viewer

• On Firestorm, the cause of this bug is the fix for Worn transparent prims/meshes no longer show with "Highlight transparent" #2577

• Reverting this commit on Firestorm fixes the bug: 49c3eed

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Firestorm Fix: FirestormViewer/phoenix-firestorm@77b07f2

https://secondlife.canny.io/admin/board/bug-reports/p/alpha-blend-rigged-mesh-turns-invisible-when-highlight-transparent-is-enabled

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[canny]

This issue has been linked to a Canny post: Alpha blend rigged mesh turns invisible when "Highlight Transparent" is enabled 🎉

[Hecklezz]

Just some additional information on the issue-
With the commit causing the problem reverted, everything works as intended until you zoom your camera out far enough that some update (seemingly LOD update) is applied to your avatar. Once you zoom back in, the same issue that you see with the commit in place also occurs. However, a different bug occurs after zooming out and back in, which is that turning off highlight transparent the mesh doesn't visually fix it, requiring you to zoom out and back in again. With the commit in place (causing the mesh to be visually wrong immediately), zooming out and back in and then turning off highlight transparent correctly displays the mesh again and doesn't require an additional zoom out and in.

So it appears there is a couple of bugs happening here whether or not the commit is in place. One exhibits issues right away, but fixes upon immediately turning off highlights. And the other only shows issues after a far zoom out, and which makes it not fix after turning off highlights requiring another zoom out.