Fix flex shrink sizes when the first pass freezes items

[intergalacticspacehighway]

Fixes #2038
Fixes #1651
Fixes #931
Closes #2039
Closes #1855

Thanks to @sebholl for the analysis and the reproduction in #2038.

Issue

When a flex line overflows, the first pass freezes the items that hit their minimum size and removes them from the shrink total. The second pass then divides the remaining free space by that total.

The total was computed by subtracting the frozen items flex shrink factors from the float total. When every item freezes, the total should become 0, but float subtraction leaves a small residual.

Example

All items have flexShrink: 1:

row (width: 350, flexDirection: row)
├── A  width: 100.1  minWidth: 90
├── B  width: 200.3  minWidth: 190
└── C  width: 300.7  minWidth: 290

The items need 601.1 but the row is only 350, so they must shrink by 251.1. Each item would go below its minWidth, so the first pass freezes all three (at 90 / 190 / 290) and removes them from the shrink total. With no items left, the total should be 0. But in float, -601.1 + 100.1 + 200.3 + 300.7 = 0.000061.

That is above the 1e-6 guard added in #1974 (for #1665), so the second pass divides the remaining free space by it: -220 / 0.000061 ≈ -3.6 million. Instead of staying at their min, the items grow to about 361M / 722M / 1084M (expected 90 / 190 / 290).

Solution

Instead of subtracting the frozen item's flex shrink factors, sum the factors of the unfrozen items and use that. This removes the float residue when subtracting values. The sum would be exactly 0 if all the items are frozen. Because the total is now exact, the second pass checks == 0 instead of the 1e-6 guard from #1974. In that case it uses childSize = 0, so the clamp sets each item to its min. This also fixes #931, where basis * (1 - flexShrink) stayed above the min when flexShrink < 1.

[meta-codesync]

@cortinico has imported this pull request. If you are a Meta employee, you can view this in D126038629.

[intergalacticspacehighway]

@cortinico Thanks for importing. Facebook Internal builds & tests seems to be failing. Could you share which tests fail?

[cortinico]

Facebook Internal builds & tests seems to be failing. Could you share which tests fail?

Those were noisy signals. It's queued for landing now @intergalacticspacehighway 👍