Hey dcadjust. So as far as I know it is fixed. Here's the steps I take to export every time. I don't honestly know if it does anything or if they fixed the issue. I guess if you are still experiencing it then its still happening though.
So. As far as I know this happens due to the greenworks plugin not playing nice with NWjs. At least that's what I remember being the issue.
First you go here. construct.net/en/make-games/addons/84/greenworks. Download the most recent greenworks plugin if you don't already have it. Make sure the version of greenworks matches your export version of NWjs and Steamworks. (This seems to rarely change so you probably won't have to do it often.) Install this plugin, restart construct to make sure it takes.
Next, go here. greenworks-prebuilds.armaldio.xyz
From that site you can use the section on the left (click the three bars) to select various versions of the greenworks.node file. You want to find the ones you need and download them. You will probably always want the most recent version (89 at time of writing). When you export as NW.js on the last step where you pick which version of NW.js to use the same area will say "Chromium 'number'", replacing 'number' with the chromium version. On that above website you want to match versions with that number.
Your runtime will be nw.js. And last your architecture, depending on what versions you are exporting, Win64, Win32, Mac, or Linux you will need to download all the different versions, or just the ones you need. Also of course match to the steamworks release you are using.
Okay so now that you have all your files there is one thing you have to do before you actually click export in construct. In the command line options box type "--in-process-GPU" without the quotes. I don't think this is related to the issue, but its in my notes so I'm adding it just in case. From what I remember, this command makes using shift tab in your game function properly.
Anyway, after all of that, you should have your exports and all the node files. So here comes the tedious bit (as if this wasn't already stupidly tedious). Before I continue I should mention that if you don't have it you'll probably want to get winRAR or an equivalent. (https://www.win-rar.com/start.html?&L=0) the free version works fine. Its just a zipping program like 7zip, it will basically help with looking through zipped folders.
First unzip your node files that you downloaded. Do NOT unzip your game. Next you'll go to your exported game (the zipped version) and double click it, which if you have winRAR should open it up so you can see inside. I'll use a Windows 64 version for this example, but I'll list stuff for the other exports below (most are the same anyway). So you will see all your files within the zip folder. First, locate the "package.nw" file and double click to go into it.
Within the package.nw file there will be a ton of stuff. You are first looking for a file called "package.json". Double click to open it in notepad. You'll find some gibberish, near the bottom should be a bit of text that says "--enable-node-worker" without the quotes. Delete that text and save the file. Don't touch anything else, you'll probably make something explode.
Okay so back in package.nw you will see several files called "greenworks-"insert OS architecture here".node. Remember those node files from before? You are going to first delete the relevant file from package.nw. So in the case of Windows 64 delete the one that says "win64" at the end. Then you are going to take your shiny new node file and drag and drop it in. If winRAR ever bothers you with questions along the lines of "are you sure" just say yes. General rule of thumb for this, always tell winRAR yes, unless it asks you to buy it cause nobody has time for that.
I'm going to go out on a limb and assume you already know all the other tidbits about adding your steam appID and the proper dlls and all that. So you're done. Close the winRAR windows, say yes to anything winRAR asks and then upload as usual.
If you need to export for Mac look below. Windows 32 bit is the same just replace 64 with 32. Linux is also laid out the same as the windows ones in terms of finding package.nw.
Path to "package.nw" in Mac
Open normal zip > mac64 > gameName.app > Contents > Resources > app.nw (at the bottom)
Where you are at now is the equivalent of package.nw, just named different, follow the steps above.
I recommend testing all this stuff before you just put it on Steam because I'm pretty sure if you screw it up, especially the node files part, the game just black screens on launch every time. Anyway, hopefully that helps. I don't know if I've ever gotten confirmation that it 100% works, but no one has mentioned a black screen issue to me since I started doing this so I'm assuming it is fine. Good luck bud, hope your game does well.