EDIT: See comment below, there's a helper method that I missed and crafting quality can be done with a 1-liner!

First thank you so much for implementing this! It works pretty good!

Having played around with it for a bit I've noticed a couple things:

1. When an item with quality appears for the first time and it reuses a frame that didn't have quality the frame does not update until the button states update (something gets added or removed)
   
   (The Hochenblume in that screen shot is quality 2 but it isn't displayed)

2. It looks like the overlay frame is generated net new even if one already exists

I was able to fix the second one pretty easily with adding a few conditional checks:

    if Baggins:IsRetailWow() and p.EnableItemReagentQuality then
        local itemQuality = link and (C_TradeSkillUI.GetItemReagentQualityByItemInfo(link) or C_TradeSkillUI.GetItemCraftedQualityByItemInfo(link))

        if not button.ProfessionQualityOverlay then
              button.ProfessionQualityOverlay = button:CreateTexture(nil, "OVERLAY")
              button.ProfessionQualityOverlay:SetPoint("TOPLEFT", -3, 2)
              button.ProfessionQualityOverlay:SetDrawLayer("OVERLAY", 7)
        end

        if itemQuality then
            local atlas = ("Professions-Icon-Quality-Tier%d-Inv"):format(itemQuality)
            button.ProfessionQualityOverlay:SetAtlas(atlas, TextureKitConstants.UseAtlasSize)  
        end

        button.ProfessionQualityOverlay:SetShown(true)
    end

I can't figure out what keeps resetting the visibility state of the buttons though. If I toggle reagent quality off and on they all show up and work until the button visibility state changes

I think this is working now so I am going to go ahead and close this.

Thank you for all your help!

Did some more digging and there's a helper for this that I missed! DOH

The whole thing can be reduced to

    if Baggins:IsRetailWow() and link and p.EnableItemReagentQuality then
        SetItemCraftingQualityOverlay(button, link)
    end

Works perfectly so far in my testing!

edit:
One issue is toggling the option needs a reload currently with using the helper since there isn't a call to ClearItemCraftingQualityOverlay(button) anywhere

Thank you for this!
Updated: 1e42350

Let me know if you have any other issues/updates.

I don't know if its just the new one liner you posted but I am not seeing the quality for items like gear.

Oh interesting... It works for profession gear (which is what I had used to test), but not actual crafted armor... let me poke at that.

edit: stranger still it works once an item moves...

Even weirder it works with potions and everything else... just not gear.

Figured it out.

Apparently item quality only shows in your bags (even the blizzard ones) if a profession window is open!

First image is the default bag frame with a profession open, second is the same default bag frame just after i closed the profession window.

Personally I think that's not terribly useful. Thankfully, there's an override!

This makes them show up all the time:

    if Baggins:IsRetailWow() and link and p.EnableItemReagentQuality then
        button.alwaysShowProfessionsQuality = true
        SetItemCraftingQualityOverlay(button, link)
    end

edit:
Which is why it was working for me, I was opening alchemy to grab an item with quality... which was triggering all the frames to update their quality overlays...

ah, nice! I have added it in as an option so you can have it either way. Always or when profession is open.