Sadly this hack is pretty bad and doesn't actually make them real alpha masks and neither does it fix the alpha issues completely, lighting and shadows are still wrong on them, along the usual other issues, it does however prevent this sorting issue in your case (and most other furniture) because these are items that use a texture that is completely solid but contains an alpha channel (probably by accident). Prior to the implementation of Materials and thus the ability to set Alpha Modes, we couldn't change surfaces like that, we had to accept whatever happened but LL implemented a "hack" that attempts to auto-alpha mask surfaces to prevent this issue from appearing. After this, you should contact the creator again and tell him, or probably link this answer so he/she knows as well and can fix all his/her content. This is all fine and dandy but the real issue here is that most people don't know these Alpha Modes exist, nor what they do or what they should be used for.
#Black dragon viewer how to edit item manual
Another solution, and the best one would be from creator side, simply not including alpha channels into textures that don't need them, this saves bandwidth and prevents this issue outright, requiring no manual intervention of the user or creator after upload. For items that need some simple alpha like plants, grass, tree leaves you can use Alpha Masking mode and set a Mask Cutoff value that compliments the effect you want to achieve. For all solid objects that do not need alpha, such as walls, your cupboards, non-transparent tables, chairs etc its quite easy, you can simply set the Alpha Mode to "None" and eliminate alpha altogether. We as users can do this via Edit - Texture - Alpha Mode dropdown. A solution to this issue is using 1Bit Alphas, also called Alpha Masks in Second Life, these Alpha Masks contain only an on/off for transparency which makes them able to be handled like solid surfaces, which in turn means we can render them as normal objects and include them in all the fancy advanced lighting rendering steps, a solid object also doesn't have the sorting issue. One of the many issues of alphas is also that they have sorting issues, sometimes depending on the camera angle a surface that is positioned behind another is rendered in front (this is what you are seeing there) and it happens universally across all Viewers in varying strengths, some objects do it constantly, some do it in certain angles only, some don't do it at all, it depends on your settings, camera angle and how the item is designed. We try to fake alphas to look like normal objects but that doesn't work perfectly, it's impossible without some really sophisticated solutions that would require completely rewriting large parts of how alphas are rendered. This is why alphas are lit differently than normal objects even if they use the exact same texture and are put next to each other. Second Life renders alphas completely separately from the main rendering method, which means they are essentially placed into the image without gaining any benefits of the actual rendering process, they lose advanced lighting and everything that comes with it, such as SSAO, the light blur pass (for soft shadows) and some other things. In 3D rendering alphatransparent surfaces are handled differently than normal solid surfaces, Second Life is no different in this case. This is NOT an issue only seen in Black Dragon and it must be fixed on the content's side.