Good work. Thanks for your contribution.

Alignment enum bugs exist where drag direction is inverted on horizontal, vertical or both axis depending on alignment:

TopLeft=OK
CenterLeft=OK
BottomLeft = Vertical drag axis inverted.
TopCenter = OK
Center = OK
BottomCenter = Vert drag axis inverted
TopRight = Horizontal drag inverted
CenterRight = Horizontal drag inverted
BottomRight = Both drag axis inverted.

I'm not a Java programmer but your ACTION_MOVE switch statement seems to look fine, and obviously works fine gravity set to center etc. It is a weird gravity issue, possibly a bug between window manager and the flutterview? Out of my depth sorry.

To rectify in the short term in your onTouch method if you can't find the cause :
int xx = params.x + (int) dx; (subtract dx instead on problematic right gravity)
int yy = params.y + (int) dy; (subtract dx instead on problematic bottom gravity)

Also there is a screen dims / vertical centering issue. If you pass a full screen colored container from flutter you might see the screen centering too far north, off the screen and padded at the bottom. NavBar size calculation issue??