Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig (#57015) by adityasharat · Pull Request #57015 · facebook/react-native

adityasharat · 2026-06-01T09:45:25Z

Summary:
X-link: facebook/yoga#1966

Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a YGConfig, every flex item whose main-axis min-{width,height} is undefined receives a content-derived floor — min(min-content, specified-size, max-size) plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor.

How to opt in: clear the new YGErrataMinSizeUndefinedInsteadOfAuto errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to YGErrataClassic so consumers using that constant continue to get the same default.

Three ways for a node to declare its min-content size along an axis, in this precedence order:

Static per-axis value via YGNodeSetMinContentWidth/Height(node, value). Pass YGUndefined to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis).
Dynamic callback via YGNodeSetMinContentMeasureFunc(node, fn). Mirrors YGMeasureFunc's shape. The algorithm invokes it for measure-func leaves when no static value is set.
Default fallback: the existing YGMeasureFunc invoked with AtMost 0, which text measurers naturally answer with their longest-word width.

For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's computeMinContentSize. This replaces the previous calculateLayoutInternal(performLayout=false) re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need.

Per-item opt-outs follow the CSS spec: explicit min-{width,height} (including 0), display: none, and overflow != visible.

Cost when off (default): a single errata-bit check per flex line plus one FloatOptional short-circuit per boundAxisWithAutoMin call. Sub-microsecond.

Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra measure() call; container subtrees do one recursive sum/max walk over in-flow descendants.

Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after xplat/flexlayout/'s MinSizeUndefinedInsteadOfAuto errata path.

Changelog:
[General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new YGErrataMinSizeUndefinedInsteadOfAuto errata bit on YGConfig. Adds YGNodeSetMinContentWidth/Height for static contributions and YGMinContentMeasureFunc for dynamic ones.'

GH pull request
#57015
facebook/yoga#1966

Reviewed By: fbcbl

Differential Revision: D105720159

meta-codesync · 2026-06-01T09:45:40Z

@adityasharat has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105720159.

Summary: X-link: facebook/react-native#57015 Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook/react-native#57015 facebook#1966 Reviewed By: fbcbl Differential Revision: D105720159

…ook#57015) Summary: Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook#57015 facebook/yoga#1966 Reviewed By: fbcbl Differential Revision: D105720159

Summary: X-link: facebook/react-native#57015 Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook/react-native#57015 facebook#1966 Reviewed By: fbcbl Differential Revision: D105720159

…ook#57015) Summary: Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook#57015 facebook/yoga#1966 Reviewed By: fbcbl Differential Revision: D105720159

Summary: X-link: facebook/react-native#57015 Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook/react-native#57015 facebook#1966 Reviewed By: fbcbl Differential Revision: D105720159

…ook#57015) Summary: Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook#57015 facebook/yoga#1966 Reviewed By: fbcbl Differential Revision: D105720159

Summary: X-link: facebook/react-native#57015 Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook/react-native#57015 facebook#1966 Reviewed By: fbcbl Differential Revision: D105720159

…ook#57015) Summary: X-link: facebook/yoga#1966 Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook#57015 facebook/yoga#1966 Reviewed By: fbcbl Differential Revision: D105720159

…ook#1966) Summary: X-link: facebook/react-native#57015 Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook/react-native#57015 facebook#1966 Reviewed By: fbcbl Differential Revision: D105720159

react-native-bot · 2026-06-01T17:09:39Z

This pull request was successfully merged by @adityasharat in 7f396d1

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meta-codesync · 2026-06-01T17:09:46Z

This pull request has been merged in 7f396d1.

Summary: Pull Request resolved: #1966 X-link: facebook/react-native#57015 Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor. How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default. Three ways for a node to declare its min-content size along an axis, in this precedence order: 1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis). 2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set. 3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width. For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need. Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`. Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond. Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants. Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path. Changelog: [General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.' GH pull request facebook/react-native#57015 #1966 Reviewed By: fbcbl Differential Revision: D105720159 fbshipit-source-id: 57a14408b10535fd84623b4d99ae9b5fba7b35ba

facebook-github-tools · 2026-06-01T22:59:28Z

This pull request has been reverted by d659203.

meta-cla Bot added the CLA Signed This label is managed by the Facebook bot. Authors need to sign the CLA before a PR can be reviewed. label Jun 1, 2026

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adityasharat requested review from cortinico and mateoguzmana June 1, 2026 09:57

adityasharat mentioned this pull request Jun 1, 2026

Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig (#1966) facebook/yoga#1966

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meta-codesync Bot changed the title ~~Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig~~ Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig (#57015) Jun 1, 2026

adityasharat force-pushed the export-D105720159 branch from 78d75a7 to f74ac72 Compare June 1, 2026 10:57

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meta-codesync Bot closed this in 7f396d1 Jun 1, 2026

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Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig (re-land) (#1966) facebook/yoga#1967

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Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig (#57015)#57015

Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig (#57015)#57015
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