diff --git a/components/layout_2020/flow/construct.rs b/components/layout_2020/flow/construct.rs
index c02dcd08fd77..896293831bcb 100644
--- a/components/layout_2020/flow/construct.rs
+++ b/components/layout_2020/flow/construct.rs
@@ -393,7 +393,7 @@ where
                     style.clone(),
                     display_inside,
                     contents,
-                    ContentSizesRequest::inline_if(style.inline_size_is_auto()),
+                    ContentSizesRequest::inline_if(!style.inline_size_is_length()),
                 ),
             ))
         };
@@ -590,7 +590,7 @@ where
                     &style,
                     ContentSizesRequest::inline_if(
                         max_assign_in_flow_outer_content_sizes_to.is_some() &&
-                            style.inline_size_is_auto(),
+                            !style.inline_size_is_length(),
                     ),
                 );
                 if let Some(to) = max_assign_in_flow_outer_content_sizes_to {
@@ -607,7 +607,7 @@ where
             } => {
                 let content_sizes = ContentSizesRequest::inline_if(
                     max_assign_in_flow_outer_content_sizes_to.is_some() &&
-                        style.inline_size_is_auto(),
+                        !style.inline_size_is_length(),
                 );
                 let contents = IndependentFormattingContext::construct(
                     context,
diff --git a/components/layout_2020/flow/float.rs b/components/layout_2020/flow/float.rs
index 2acc20950045..df9cbbaf5145 100644
--- a/components/layout_2020/flow/float.rs
+++ b/components/layout_2020/flow/float.rs
@@ -33,7 +33,7 @@ impl FloatBox {
         display_inside: DisplayInside,
         contents: Contents<impl NodeExt<'dom>>,
     ) -> Self {
-        let content_sizes = ContentSizesRequest::inline_if(style.inline_size_is_auto());
+        let content_sizes = ContentSizesRequest::inline_if(!style.inline_size_is_length());
         Self {
             contents: IndependentFormattingContext::construct(
                 context,
diff --git a/components/layout_2020/positioned.rs b/components/layout_2020/positioned.rs
index 8b5f078f520c..9459f3c3f849 100644
--- a/components/layout_2020/positioned.rs
+++ b/components/layout_2020/positioned.rs
@@ -60,7 +60,7 @@ impl AbsolutelyPositionedBox {
         // "Shrink-to-fit" in https://drafts.csswg.org/css2/visudet.html#abs-non-replaced-width
         let content_sizes = ContentSizesRequest::inline_if(
             // If inline-size is non-auto, that value is used without shrink-to-fit
-            style.inline_size_is_auto() &&
+            !style.inline_size_is_length() &&
             // If it is, then the only case where shrink-to-fit is *not* used is
             // if both offsets are non-auto, leaving inline-size as the only variable
             // in the constraint equation.
diff --git a/components/layout_2020/style_ext.rs b/components/layout_2020/style_ext.rs
index 59c87fd633c4..67d8bc78d7eb 100644
--- a/components/layout_2020/style_ext.rs
+++ b/components/layout_2020/style_ext.rs
@@ -40,7 +40,7 @@ pub(crate) enum DisplayInside {
 }
 
 pub(crate) trait ComputedValuesExt {
-    fn inline_size_is_auto(&self) -> bool;
+    fn inline_size_is_length(&self) -> bool;
     fn inline_box_offsets_are_both_non_auto(&self) -> bool;
     fn box_offsets(&self) -> flow_relative::Sides<LengthPercentageOrAuto>;
     fn box_size(&self) -> flow_relative::Vec2<LengthPercentageOrAuto>;
@@ -52,14 +52,14 @@ pub(crate) trait ComputedValuesExt {
 }
 
 impl ComputedValuesExt for ComputedValues {
-    fn inline_size_is_auto(&self) -> bool {
+    fn inline_size_is_length(&self) -> bool {
         let position = self.get_position();
         let size = if self.writing_mode.is_horizontal() {
             position.width
         } else {
             position.height
         };
-        size == Size::Auto
+        matches!(size, Size::LengthPercentage(lp) if lp.0.as_length().is_some())
     }
 
     fn inline_box_offsets_are_both_non_auto(&self) -> bool {