So these particular containers have `flex-wrap:wrap`, and when the rounding works out such that things don't quite fit, the last flex item wraps to a second line (and then both the first and second lines have their content centered, per `justify-content:center`). I'm tracking down the arithmetic in the markup that results in 1px of overflow...
Bug 1923245 Comment 8 Edit History
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So these particular containers have `flex-wrap:wrap`; so when the rounding works out such that things don't quite fit, the last flex item wraps to a second line (and then both the first and second lines have their content centered, per `justify-content:center`). I'm tracking down the arithmetic in the markup that results in overflow...