To all the modders here.

The conflict arises from discrepancies between different versions of GSON. My suggestion would be to remove your dependency on GSON 2.8.6 and opt for a different approach. Instead of using the static method JsonParser.parseString, you can create a JsonParser object and then use the parse method.   JsonParser jsonParser = new JsonParser(); jsonParser.parse(jsonString)  

On the download page, click the button for "Show all versions", point at the little i next to the Installer link, and use the Direct Download link that pops up. That should let you download it if you have issues with the adfocus thingy.

I was downloading Forge's installer for the latest Minecraft version. The official Forge page led to the familiar "press skip after countdown" ad page I've gone through many times. This time, the "Skip" button just never showed up after the countdown finished. The page for the ad was clearly bugged, because it showed as a blank white page up to the rows of pixels right at the top, where I saw a sliver of the actual page 2 or so inches tall but with the width of the whole page. I tried different versions (1.20.4 latest and 1.20.2 recommended), refreshing, closing the tab and going there again, to no avail. Then this Forum's signup Security Check wouldn't let me through. I rebooted my PC and now it did work just fine. But the download still doesn't (still no Skip button), even after booting my PC. The the ad was seemingly bugged as well, because it showed as a blank white page up to the rows of pixels right at the top, 2 or so inches tall but the width of the whole page. I even manually scrolled down to its bottom and back up, but that didn't fix it either. I sent a report of the same issue to Adfoc.us, but that led to a page labeled "Just a moment..." that doesn't lead anywhere after 10mins of waiting. Refreshing the page does nothing, and there is no confirmation message in my email. So I don't even know if the report to AdFoc.us went through. I'm stumped.

Did some more testing and found workaround. Newest MC and Forge for it didn't have this problem, also isolated that problem is only with fullscreen, meaning there is problem with exclusive fullscreen handling when application gets moved to background/minimized. So installing this mod fixed problem for MC 1.12.2 and forge for it https://www.curseforge.com/minecraft/mc-mods/fullscreen-windowed-borderless-for-minecraft

Also remove Butchercraft