The off-screen-layout bug (nodes centred on the scrollable width instead of the
viewport) only reproduced at the default window size, so a single-width test
would have shipped it. Verifies the graph stays framed at 1920/1400/1100/820,
and records which assumption in the pixel census is safe: the measurement uses
the canvas buffer size, not the screen size, because the CSS min-width makes
narrow viewports scroll.
