This test makes sure that underlines contribute to visual overflow properly. The SVG font has a descent of 0, which means that underlines will lie outside of the glyph boundaries. The test makes sure of two things:
  1. (Left box) That moving a decoration that contributes to overflow from the top of a line to the bottom is redrawn as if it was always on the bottom side
  2. (Right box) That moving a decoration that contributes to overflow to a position where it does not contribute to overflow anymore redraws as if it had never contributed to overflow
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