Margin when using i3 tiling WM

When running the spinning-square example, the square isn't centered in the window and, when scaling down the window, margins will be visible at the right and top edge of the window like this:

The problem won't occur in xfwm4

The problem also won't occur if the window is initialized in floating mode (if i3 is configured to open windows in floating, which makes little sense for a tiling wm). The problem however persists if already opened windows are put into floating mode.
The problem won't appear with the glutin examples so it's not a pure glutin issue.

The exact same problem will occur with all the examples in the piston-examples repository

I don't know if the behaviour of tiling wms to ignore the window size requests of clients is standard but it would be nice for piston to work without problems anyways.

Start with the spinning-square example in the Piston-Tutorials getting-started example and reproduce the behavior under i3, comparing it with xfwm4 and the glutin examples. Then inspect how the Piston examples initialize and size their windows. Done means the square is centered and no right or top margins appear when resizing under i3.