Refactor mobile menu scrolling behaviour

[roundaboutluke]

Fixes an issue on Android Chrome where the menu behaved incorrectly when the viewport height changes as the address bar shows/hides.

The bug was reproduced in Android Studio emulator and fix was tested there also please test on a real device to confirm its resolved.

iOS Safari remains working with these changes. Android Firefox has not been tested.

Note: there is still an existing issue on Android Chrome where if the address bar is hidden and the menu is closed via the X button then the address bar does not reappear. This behavior actually existed prior to this change (and my last PR from testing) and is not introduced here.

[ccev]

sorry to say but this didn't really help much, i've just pushed a change where some offsetting issues are gone, but the sticky title still juggles around while scrolling. also now the bottom padding is fucked up.

i also noticed you removed the extra scout menu styling; admittedly it's a bit of a hack, but now the scout menu styling is also fucked up.

[roundaboutluke]

Might be best to leave this for now - I've got a friend bringing me an old android soon, I'll be able to test on a real device then

Annoying that I could replicate in the Android Studio emulator and as you saw got it perfect there so a real device to get it right is the only way forward I think (side by side with iPhone to ensure compatibility with both that is!)