One 1fr is a share of what is left, and it has a floor.

200px 1fr 1fr in a 1000px container with a 24px gap. The two gutters take 48px and the fixed track takes 200px, so the two fr tracks divide 752px — 376px each. Not 333.3px, which is an equal share of the container. Not 400px, which is an equal share of what the sidebar left. And if one of them holds something 420px wide, it is clamped up to 420px and the other drops to 332px to pay for it.

The engine is free and needs no account. Paste the row — one track per line, in the order they appear. A bare 1fr is minmax(auto, 1fr), so its minimum is its content's min-content width and not zero: a track that will not fit is clamped past its share, the others pay, and once they have nothing left to pay with the row is wider than its container by an exact number. Hiding a part is not a redistribution — it is the whole row re-resolved, it removes a gutter as well as a track, and where its width goes depends entirely on what is still flexible. Hide the last flexible track and the space is claimed by nothing at all. And flex: 1 is not flex-grow: 1: the shorthand expands to flex: 1 1 0% and every item starts at zero, while the longhand leaves flex-basis: auto and every item starts at its own content. Every lane ships a worked example that costs nothing. Writing the report needs credits.