A note about using a mouse on an emulator

Short version:

Press Ctrl+F5 to confine the host system mouse to the extents of the emulated screen

Long version:

The cursor position resets to the middle every level. This is intentional and the developer addresses it in the version notes.

The issue on emulation is that this 'reset' mechanism isn't pulled through to the host system.

So, when the ship position resets after clearing a round, your host mouse might be way out in wherever-the-heck-you-left-it-last-land, but the game tells itself the mouse is in the middle. The instant you move the mouse, the emulator whips over to the OG system position (sometimes in the nether regions beyond the emulated screen). The triangle (chevron? marker? arrow?) indicator below the ship whips over there too. Meaning the ship immediately goes in that direction whether you like it or not. This can be disastrous if you need to dodge nasties in a hurry.

Out of habit, I move the chevron to the central mark after clearing a round¹, during those few seconds between clearing the last enemy and seeing the 'level complete' screen. That way, the mouse position is the same on both host OS and emulation, and the 'reset' has no practical effect.

¹ The exception is challenge rounds. When the challnge starts the mouse is not reset. Your ship position at <end of level> is the same as <start of challenge round>. So if you have the challenge medal, get your ship into prime position for best performance.

In any case, it turns out there's a very easy solution. There's a toggle on emulators (tested on Sheepshaver and Basilisk II) that allows you to preempt all of this by pressing: Control + F5

This "grabs" the mouse, essentially syncing up the mouse position in both host and emulator. The mouse is now confined to the extents of the emulated screen. Et voila!

Problem solved. Robert's your cousin's father.