when using noCursor(), and mouse is not pressed, hitting F10 causes the
cursor to re-appear. clicking twice it usually dissappears again. This
does not happen when the mouse is pressed while F10 is pressed.

I believe it's due to windows default keyboard shortcut for F10:
"Activate the menu bar in the active program."

still- there must be some way to disable or prevent this (as there is no
menu bar for processing applications)

the following code (modified from an example) shows the bug:

void setup() {
noCursor();
size(200, 200);
background(102);
}

void draw() {
stroke(255);
if(mousePressed) {
line(mouseX, mouseY, pmouseX, pmouseY);
}
}

this is an os problem, not something we can get at through java.