I just upgraded my allow demo to 12.9.0 and the scroll up to reveal page header is working for me.

I am missing a few packages for hosting and DXP, this is my Alloy demo config, it would be strange for those other packages I would think to affect that tho.

    <PackageReference Include="EPiServer.CMS" Version="12.9.0" />
    <PackageReference Include="EPiServer.Find.Cms" Version="14.1.0" />
    <PackageReference Include="Wangkanai.Detection" Version="5.2.0" />

Have you tried fully clearing all cache etc etc just in case there's been any chance in the JS that's caching?

You might be right. I tried clearing cache, I can now trigger the toggle of pageheader "somethings". I have only tired this so fare in my dev env.

Cache doesn't seem to be the issue, since I still have the error. 

Are there any errors in the console window?.

Now I run 12.10.0 and the problem still exist. I didn't see this console error earlier in 12.9.0, so It might not be any link between the two, but this is what I see in the console window. 

Source: Episerver/shell/clientresources/12.10.0/epi/epi.js:2

SyntaxError: Unexpected token '}'
    at dojo.fromJson (dojo.js:15:82234)
    at Object.json (dojo.js:15:120242)
    at _497 (dojo.js:15:123066)
    at _2cd (dojo.js:15:73012)
    at _2cb (dojo.js:15:72704)
    at resolve.callback (dojo.js:15:73525)
    at dojo.js:15:125114
    at _2f2 (dojo.js:15:75368)
    at _2ec (dojo.js:15:75232)
    at _2f1.resolve (dojo.js:15:76693)

try network tab - do you see any error requests, what do they say?