To replicate:

1. Download Furance for macOS (Apple silicon)
2. Open Furnace (may require overriding certain unidentified developer permissions, not unusual on a Mac)
3. Click the "settings" tab
4. Click the "full screen" button
5. You should be immediately brought into full screen mode. Now the cursor should act on the screen lower than it appears (appears to be around two rows lower in the pattern table). You should be unable to bring up the Dock or bring down the Menu Bar by moving your cursor to the bottom or top of the screen. Even when highlighted by the "cursor" (below where the cursor appears to be), none of the tabs in the top row should work. To exit full screen mode, swipe up on the trackpad with three fingers, move your cursor (which should now be working normally) to hover over the Furnace "Desktop selector" at the top until a circle with two inwards facing arrows appears. The cursor displacement may be different now when over the Furnace window but should work normally outside of it.
6. Even when quitting Furnace or even restarting the MacBook, the cursor displacement issue should persist

To fix:

1. Quit Furnace if you haven't already (if you are in an unsaved file, choosing the button for whether to save may be difficult due to the cursor displacement...)
2. Open a Terminal window and navigate to the Application Support folder via "cd /Library/Application Support" or a similar command
3. Run "rm -rf Furnace" (or just remove the "furnace.cfg" file, this works too)
4. Reopen Furnace. The intro animation/music should play and the cursor should work as normal again

I hope the above steps are clear and work for others trying to replicate the issue. I have been able to replicate and resolve on my own MacBook multiple times, so it is consistent for me. Entering full screen mode via the typical green circle in the top left works normally, so this appears to be some settings/config issue when using that specific button within Furnace.