Design guidelines

Don't clutter the widget code with drawing primitives

The widget implementations should contain high level code only. Low level drawing primitives should be in separate packages. That way the widgets remain easy to understand, enhance and test.

E.g. the gauge widget contains code that calculates the size of the rectangle that needs to be drawn. It doesn't contain code that draws the rectangle itself as that belongs into the draw package.

Provide test helpers for all functions in the draw package

To simplify unit tests of widgets, a test helper should be provided to all functions in the draw package.

E.g. a function called Rectangle() that draws a rectangle should come with a helper caller MustRectangle(). Tests of a widget that uses Rectangle() can just specify the expected rectangle by calling MustRectangle() on the test canvas.