Canvas Error when scrolling off desktop

BlitzPlus Forums/BlitzPlus Programming/Canvas Error when scrolling off desktop

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I am having a really bizarre problem. I am creating an app which interfaces to a series of spreadsheets and does a number of operations on them.

When you select a spread sheet from a list it creates a window and uses a canvas to create a custom list box.

Im almost finished and everything was going fine until i came across this weird bug

If you drag any of the windiows about until part of the canvas is off screen the canvas suddenly is moved out the window and onto the desktop (top left corner)

Its really bizare and ive no idea how to remedy it, any ideas?