I hear what you’re saying Hugo, but when you do get round to it (and in answer to @tristan & @tomtom’s question)… A really great implementation of the auto-hiding context/FAB button is used by Viber on Android (v.184.108.40.206). It slides out of view at the same speed you scroll down - better than the crappy YouTube one that just disappears in a puff as you scroll down, and doesn’t re-appear again unless you scroll all the way back up to the top (so it’s easy to ‘lose’ it).
They use it for often used things too, like ‘new chat’ while on the chat screen, or ‘new contact’ while on the contacts screen.
The Viber app also employs the cool trick simultaneously sliding away the top half of the navigation bar and the context/FAB button in opposite directions, as soon as you scroll down. It immediately gives more screen real estate and looks pretty slick too. In a similar vein to your transaction detail view. Oh and they slide back into view as soon as you scroll up too.
If you’re worried about people ‘losing’ the button, perhaps you could consider sliding it out of view at half the speed at which the user scrolls down?