Alright then, from the FrameArtDefs.xml file... this section might be related, i guess.

----
<UIArtDefine>
    <InternalName>BlackFadeMaskLeft</InternalName>
    <Define>
      <UISize>Small</UISize>
      <ResizingData>
        <HorizontalStretching>true</HorizontalStretching>
        <VerticalStretching>true</VerticalStretching>
        <X1>200</X1>
        <X2>250</X2>
        <Y1>8</Y1>
        <Y2>14</Y2>
      </ResizingData>
      <State>
        <Type>Up</Type>
        <TextureData>
          <Texture>GC3_S_Top_Side_Blend_Mask.png</Texture>
          <MirrorHorizontal>true</MirrorHorizontal>
          <DefaultAlpha>.85</DefaultAlpha>
        </TextureData>
      </State>
    </Define>

----
Seems like the MirrorHorizontal instruction is failing.
???

Alrighty..

Still no luck getting to fix this weird stuff.
Soooo -- some SD staff which are in charge of HUD components & could know exactly what to do.. please help!

I'm just stuck in development and can't form a reliable design principle that would have to spread in a LOT of unique BG/FG files. Without that workaround solution.. the UI becomes almost unreadable during trade.

I think you can fix this by editing MenuArtDefs.xml. Its not doable via a mod, you would have to edit the file and re-create the games bin files with the compilers located in the tools folder.

Or you can change/replace GC3_Startup_Mask_Left.png, which I think might controll that shading?

I don't think i could reliably tackle the Bin recompiling tricks (not yet anyway) for such stuff.

But if "replacing" any GFX file could be worth a try.. i'm certainly willing and able. That would require trial&error steps to calibrate the intended structure as it relate to whatever HUD/UI states. With enough luck.. hopefully.

PS; I'm far enough into development of basic GFX assets for the upcoming X-Com_A-Liens thematic mod (65+ Factions to begin with) but i have yet to determine how to process the necessary custom XML files (data picked out from the usual generic defaults generated by the ingame civilization editor/compiler).
I know you managed to create many of the core assets for your own package(s). Any hints.. or should i rationally expect heavy manual daunting  tasks ahead?
No clear guides exist, AFAIC. Sooooo.. if you could spare a few minutes to somewhat recall those working methods, please do.

UCP_Factions from my mod should contain all the stuff you need to see, to make stuff for your custom factions, appending is a pretty easy task tbh. These you can make via the civilization builder, or by having your own MyMod_FactionDefs.xml.

Aaaarrrgghhh.. so far no luck with editing the suggested specific file. There has to be a tricky UI framework in place -- possibly within DiplomacyWnd.XML  and its wacky controls. To think that some SD staff could immediately point me at the correct editing (or repair) step is a bit frustrating for sure. Gonna have to wait, again! Thanks for the help anyway.

Secondly -- Yet.. the whole XC-AL_FactionDefs.XML (custom) file has to be manually edited from data sources which i don't have straight access to, right? Each values are pre-compiled into a sealed bunch of infos while the actual instructions aren't exposed for copy/paste tasks.  And, so on.

InteractionWnd.xml

Regards to FactionDefs, you need to look at the scheme files for the "instructions" so to speak.

Interaction seems to offer a few good hints. I'll take a look at what it's supposed to do & how it could accept slight corrections to fix what i need. Let's hope it is a simple GFX swap trick.

Once that scheme reveals the valid Data pulling tricks & various instruction mechanics.. there's still no direct way to interpret the source code for each custom Factions, shouldn't there be some de-compile process to obtain every details?
Without them.. nothing could be copy/pasted. I soooooo wished the code engine could simply auto-generate whatever is necessary beyond the basic creative steps currently offered by their (limited) designer.

Okay.. i'll just have to give up trying various edits to both XML and related GFX file(s). Nothing sticks around enough to detect any worthy changes at all.
Sooooo, let's hope someone at Stardock will be kind enough to offer a definitive solution once back from the Holidays hiatus.

Alrighty...
Now that the Holidays hiatus is over, let's hope some benevolent Stardock expert in HUD/UI concerns would find precious spare time for this stuff.
I have no choice but to suspend further GFX development for my XC-AL factions until i can reliably fix that left-side background mask or modify whatever default XML system which causes wacky proportions on a Large screen (if that matters.. when playing under UI-HD / 4K Monitor with 3840x2160 sizing conditions, btw).

Im guessing you do not keep upto date with the info the Devs share. The UI is getting a complete overhaul for the next expansion.

Oh.. good news then.
Let's hope it doesn't break some of the GFX work i've already created for that XC-AL mod though.   

I really don't want to lose all of the current design concepts like this FG style for the Aquatic factions. It just has to workaround the default HUD calibration to fit details in some tight corners instead of relying on the indirect "Movies" system to squeeze onto diplomatic components for example.


Or even this cool custom stuff for the X-Com:Enemy Unknown (by Firaxis) Outsiders & many (15) other Aliens!

https://postimg.cc/kBGVypr3

The Interaction/Trade screen needs special attention for sure.
Let's see what happens.

--

So far so good!