Reference+
Name
mouseY
Description
The system variable mouseY always contains the current
 vertical coordinate of the mouse.
 
 Note that Processing can only track the mouse position when the pointer
 is over the current window. The default value of mouseY is 0,
 so 0 will be returned until the mouse moves in front of the sketch
 window. (This typically happens when a sketch is first run.)  Once the
 mouse moves away from the window, mouseY will continue to report
 its most recent position.
Examples
void draw() { background(204); line(20, mouseY, 80, mouseY); }
Related

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