I fully understand where you appear to be experiencing difficulties with your double-tap. This is one aspect of iPadOS 26 which can, until explained and understood, cause frustration.
A quick double-tap at the top of the a full-screen view will switch to windowed view as you observe. A quick double tap at the top of an App window does the reverse - switching to a full-screen view.
Insofar as returning to the top of a long page, you still use two taps at the top of the full-screen or active App window - but need to slow down...
Tap, briefly pause, then tap again; the page will scroll to the top exactly as you are accustomed.
Windowed Apps can be freely resized by dragging from any of the four corners of the window; this is signified by the "handle" icon at bottom-right of a resizable window. A tap of the multitasking control button (represented by the traffic-light button at top-left of an App window will expand the control; tapping red will close the App and associated App window, yellow will minimise the window but leave the App running in the background, while green will maximise the App window to a full screen view.
iPadOS 26 multitasking has received a significant makeover from that to which you may be accustomed in iPadOS 18.x and earlier - and may take a little time to learn and adjust your workflows. You would be well advised to review the official iPadOS User Guide and the many available online tutorials (such as YouTube) to become better acquainted with the new multitasking interface and associated gestures and controls.
Work with multiple windows at once on iPad - Apple Support
OS - iPadOS 26 - Apple
Multitask on iPad with iPadOS 26 - Apple Support (begin with the excellent inset video)
If you dislike the windowed environment, you can switch to using the basic Full-Screen App mode - but this disables all multitasking capabilities and hide the traffic-light controls; you will not be able to use split screen or slide-over modes with which you are likely familiar:
Settings > Multitasking & Gestures - select Full Screen Apps
