Okay update for anyone that’s bothered or interested.
I created an entirely new project to confirm if what I thought I knew about how master pages work was accurate (tl;dr it is).
I created a master page, then used it to create an index.html at the same root location, then created a sub folder and a new page in there using the master page as teh template too.
Updates to the non-editable parts of the master page got pushed correctly when I clicked the round arrows and chose update whole project.
Just as the docs say and just as I thought.
So I went back to my project (remember, all the pages are empty bar a paragraph element), and tried it again (updating master and pushing) and they failed.
So I deleted all my sub pages, my (virtually finished - very annoying) index.html and triple checked my master page. All looked good.
I re-created the blank index and a blank sub page. Tried to update the master and push the changes and got a weird error this time.
I don’t know what that’s telling me, but I noticed it went away when I removed my page links from my offcanvas menu.
So is my offcanvas menu the issue?
Here is what I’ve done and my reasoning - please, someone, tell me if this is correct or if I’m wrong:
The entire point of using a master page is that I want a nav bar on every page of my site (and a footer).
So I created a custom nav bar in html & css and used the BS5 offcanvas classes to make it work.
I populated the menu with links to pages (in subfolders) that DO NOT CURRENTLY EXIST - but will exist after I save the master page and use it as a template for each of those pages.
So after creating the Master Page and the links for the offcanvas menu, I saved it and then inside a sub folder I created a new page, using that master page as the template and the new page has all the menu links now correctly updated by PG to have the ../ before links outside of the sub folder - brilliant I thought!
But now, if I make a change to the master page and click the circle arrows and update whole project I get this new error for both the index and the new sub page I just created:
and then, at the bottom of the index and sub page there is a huge block comment added by PG: