Well, in case anyone is wondering, I managed to resolve the HTML5 game breaking bug issue in the end (but it's a hollow victory...)
Nothing, literally NOTHING (even TrueValhalla's $64.95 'Mobility Engine', which I feel I've wasted money on at this point, as it didn't fix the issue at all, despite being a HTML5 fix all / scaling solution) - Nope, the only thing that worked was painstakingly going BACK through runtimes, until I hit v2.2.0.261...
I can 100% say that the issue was introduced in v2.2.1.287, as the above previous runtime works PERFECTLY for HTML scaling / canvas relocation, so really, isn't it just a case of YoYo's devs doing a diff and some manual investigation across these versions to FIX this issue they introduced? - it's quite obviously a bug introduced then, as it works, works, works, for every version prior to that (I know as I originally jumped back further), and suddenly STOPS working correctly after that version, getting worse across the 2 v2.2.1.x versions...
Seriously guys, this isn't that hard to investigate and fix, surely? - plus, it's the right thing to do given the exporter is a paid-for product!.... Even
@Appsurd 's helpful suggestion of iFrames probably won't fix the browser magnification / zoom scaling issue, but I'll certainly try it to see if it at least fixes the OFFSET issue in later runtimes, or I'll be completely stuck on the older one...
As it stands I'm now going to have to use v2.2.0.261 for ALL of my HTML work, which as you're probably aware has issues with ASSET layer elements/placements being incorrect (a historical fix for which may be related to the broken functionality in later releases?) - So I've had to remove all assets in rooms, and create objects for each. Not ideal, but hey, it gets me working again at least...
I really would like YoYo to treat this issue with the seriousness it deserves (and their purchasers of this module deserve!), as other than this, the HTML5 module is FANTASTIC, but well, YoYo have obviously (innocently and inadvertently) broken perfectly working functionality for a module that users have paid in good faith for, so it's unacceptable to not fix it!
I love you guys, but this seemingly indifferent attitude to a long-standing, game breaking bug, that's been broken and not even touched for 2+ years, is making me lose some faith in YoYo, and sadly reminds me of that 'other' badly maintained product....