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I would like Sony to add two other sizing options to snap to fit.
While in Vertical Muti-Window
Unfortunately, I think this is to do with YouTube (and others) native aspect being 16:9.
The page is snapping to size, but it's snapping to the page aspect ratio given by YouTube and others as 16:9
Only when these apps offer variable native aspects, will you see the snapping fit flush to the media size.
It's a 21:9 phone and should really have 21:9 vertical multi-window.
It's just that Sony hasn't take in consideration.
Up there I'm watching 21:9 Sony's "Jumanji: Welcome to the Jungle (2017)" Purchased on GooglePlay Store and 4:3 "Roberta (1935)"; an old Fred Astair movie.
We all know about these aspect ratios; because we lived through these screen standard changes.
Of course, 16:9 works with well in vertical multi-window, just to accommodate horizontal view getting bars... (as most point out)
Perhaps this is a Google thing that they need to handle for Android 10. Motorola Razr is a 21:9 form factor phone yet, does not have the Sony's nifty Snap; that makes multi-tasking actually manageable.
I played with a friend's no Sony budget 16:9 phone, and it can only Multi-talks in two even halves.
Update 1/5/2020
On the Xperia 1, YouTube multi-window has added a manual sliding sizing between 16:9, 4:3, to half.
There still is no 21:9 snap, but it is a good update.
This rollout is for YouTube app on all Android phones including Plixel 3. So they can split screen besides 50/50, and split screen like us Sony Xperia 1 users.
Yey!