I really need to know how much this barrier changes.
Until then I can only guess that it depends on the speed you are moving. If you are moving fast and my guess from the mail above is right, then slow movement should always allow to move to the border, while with fast movement, the border seems to vary.
Example (border at 10)
initial pos|speed|result pos
9 1 10
9 2 9 !
8 2 10
6 5 6 !
5 5 10
One belated addition: The atari mouse and amiga mouse can be used with the 2600daptor with either trackball hack with "grab mouse in emulation" if mouse sensitivity is turned up to 100%, even though this is way too sensitive if "grab mouse in emulation" is turned off. For the actual CX22/CX80 trak-balls, they also work as a mouse with "grab mouse in emulation" turned on, but even at 100% mouse sensitivity, the cross-hairs motion is about 2-3 times slower than you would want it. With "grab mouse in emulation" turned off, CX22/80 motion is acceptable, but "runs out off axis" occasionally due to Windows cursor hitting the edge of the screen and needs the opposite direction spin to re-center Windows cursor. This may be correctable by turning the mouse speed down in Windows settings. Just for summary, the usb mouse, atari mouse and amiga mouse do not work well with "grab mouse in emulation" turned off because the Windows cursor moves so much faster than the cross-hairs, even if Windows setting turns the mouse speed all the way down. These mouse trackballs might also have a lessened issue with "grab mouse in emulation" off, if mouse sensitivity is set very high and Windows mouse settings is set to minimum cursor speed, but I would recommend keeping "grab mouse in emulation" on for these mouse trackballs and reserving max trackball sensitivity in Stella and minimum cursor speed in Windows settings only if using the CX22/80 with 2600daptor.
I can conclusively say that the issues I observed are due to the game motion still being linked to (and often limited by) the Windows mouse cursor motion, which is not in sync, when "grab mouse in emulation" is turned off.
Edit: For using trackballs with Stella games with no trackballs hacks, use a CX22/80 trak-ball with 2600daptor and use joystick mode for all three components or use "Universal Control Remapping" app with USB or atari mouse axes mapped to four keyboard direction buttons. If you remap the "left-click" button on your mouse you will have to use keyboard keys to get around or find a program to substitute the left mouse button with another mouse button or keyboard key. It is possible a simple re-mapping another key/button to the left mouse button and using that key may just activate whatever key/button the left mouse button was remapped to, but not sure.
WinKey+D = allow highlight desktop icon
arrow keys = select different desktop icons
Enter = select
Alt+Tab = select different open apps
Alt+F4 = close a selected app (specifically closing UCR)
Shift+F10 = mouse right-click, shortcut menu
There does not seem to be a good universal "left-click mouse" keyboard shortcut
If things get really wacky, you may need to use Task Manager to shut UCR down or do a PC reset to get the mouse button to work normally again. There are basic tutorials for using UCR on youtube and Steam. Both CX22/80 joystick mode and UCR are a little slower than an actual trackball hack, but I think UCR is a little faster than joystick mode, perhaps because you are using a newer mouse trackball and software rather than the older CX22/80 trak-ball.
This UCR setup seemed to work well enough to get in and out of Stella and close the UCR app. YMMV. I have a "profile 1" which is empty that I switch to and save before closing. This is probably backwards as this should be the default profile. I have the left-click mouse button mapped to a fire key "a" and the middle mouse button mapped to the left-click mouse button.