When you use the "Charge the current user" action from Bubble's Braintree plugin and the button that triggers it is positioned low on the page, the PayPal pop-up appears at the same scroll position as the button rather than in the centre of the viewport. This can partially cut off the pop-up or push it out of view.
What you might notice
The PayPal pop-up appears too low on the screen and requires scrolling up to see it fully
Part of the pop-up is cut off or not fully visible
The checkout flow itself still works correctly — the pop-up just displays in an unexpected position
Workaround
Add a "Scroll to an element" action before the "Charge the current user" action. Set the element to scroll to as the page itself, with an offset of 0. This scrolls the user back to the top of the page before the pop-up appears, so it displays fully in view.