When you enumerate the VCS widgets via OSC (using message: /osc/widget,i), one of the widget responses will be an Association entry with "screenRectangle" as the key; you can use the values in the LayoutFrame to construct the screen rectangle.
Here's an example:
{
creationClass = Association;
key = screenRectangle;
value = {
bottomFraction = 0;
bottomOffset = 669;
creationClass = LayoutFrame;
leftFraction = 0;
leftOffset = 0;
rightFraction = 0;
rightOffset = 1162;
topFraction = 0;
topOffset = 0;
};
}
This describes a screen rectangle of (0 @ 0) corner: (1162 @ 669).