The bug

When an item is placed in the enchantment table, it is possible to confirm which enchantment to apply using any mouse button, be it the right button, middle button, forward button, back button, or buttons 6, 7 or 8. Mouse buttons nine and above are not recognised by the game at all (MC-266231), and therefore do not trigger this issue.

Whether or not the mouse buttons are bound to any in-game controls is irrelevant.

How to reproduce

1. Have a mouse with more than one button, and preferably more than three buttons
2. Place an item into an enchanting table, with lapis lazuli if required
3. Press the highest mouse button you have while mousing over one of the available options

Expected results

Nothing should happen.

Actual results

The item is enchanted as though you left-clicked.