Whether Alt / Option was held.
Which button changed state: 0 left, 1 middle, 2 right.
Bitmask of the buttons currently held.
Whether Control was held.
Whether Command / Windows was held.
Cursor position in untransformed window coordinates, which differs
from x whenever the canvas is being scaled to fit.
Cursor position in window coordinates, vertically.
Whether Shift was held.
Cursor position in canvas coordinates, with the window's fit undone.
Cursor position in canvas coordinates, vertically.
Payload shared by the mouse events, following the DOM's names.
🧪 Not in the HTML Canvas standard.