Determine Window Coordinates

Hey there, I am trying to program my own “window handling system” (actually just some controls like buttons and so on).
I would like to draw them with simple lines (for the borders). But I faced a problem: If I want to make a Line with 200 pixels width, this results in a very long line (i guess due to world coordinates?).
Can anyone tell me how to calculate window distances (200 px) to world distances?

thanks in advance!

Oh, found the solution by myself.
I just can set the QueBucket to Bucket.Gui in my Line2D class.

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