diff --git a/layout/generic/ReflowInput.cpp b/layout/generic/ReflowInput.cpp
index b8bff5bdbcd9..581f4371bd22 100644
--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -93,6 +93,9 @@ ReflowInput::ReflowInput(nsPresContext*       aPresContext,
   if (aFlags & B_CLAMP_MARGIN_BOX_MIN_SIZE) {
     mFlags.mBClampMarginBoxMinSize = true;
   }
+  if (aFlags & I_APPLY_AUTO_MIN_SIZE) {
+    mFlags.mApplyAutoMinSize = true;
+  }
 
   if (!(aFlags & CALLER_WILL_INIT)) {
     Init(aPresContext);
@@ -242,6 +245,7 @@ ReflowInput::ReflowInput(
   mFlags.mIOffsetsNeedCSSAlign = mFlags.mBOffsetsNeedCSSAlign = false;
   mFlags.mIClampMarginBoxMinSize = !!(aFlags & I_CLAMP_MARGIN_BOX_MIN_SIZE);
   mFlags.mBClampMarginBoxMinSize = !!(aFlags & B_CLAMP_MARGIN_BOX_MIN_SIZE);
+  mFlags.mApplyAutoMinSize = !!(aFlags & I_APPLY_AUTO_MIN_SIZE);
 
   mDiscoveredClearance = nullptr;
   mPercentBSizeObserver = (aParentReflowInput.mPercentBSizeObserver &&
@@ -1666,6 +1670,10 @@ ReflowInput::InitAbsoluteConstraints(nsPresContext* aPresContext,
     computeSizeFlags = ComputeSizeFlags(computeSizeFlags |
                          ComputeSizeFlags::eBClampMarginBoxMinSize);
   }
+  if (mFlags.mApplyAutoMinSize) {
+    computeSizeFlags = ComputeSizeFlags(computeSizeFlags |
+                         ComputeSizeFlags::eIApplyAutoMinSize);
+  }
   if (mFlags.mShrinkWrap) {
     computeSizeFlags =
       ComputeSizeFlags(computeSizeFlags | ComputeSizeFlags::eShrinkWrap);
@@ -2379,6 +2387,10 @@ ReflowInput::InitConstraints(nsPresContext*     aPresContext,
         computeSizeFlags = ComputeSizeFlags(computeSizeFlags |
                              ComputeSizeFlags::eBClampMarginBoxMinSize);
       }
+      if (mFlags.mApplyAutoMinSize) {
+        computeSizeFlags = ComputeSizeFlags(computeSizeFlags |
+                             ComputeSizeFlags::eIApplyAutoMinSize);
+      }
       if (mFlags.mShrinkWrap) {
         computeSizeFlags =
           ComputeSizeFlags(computeSizeFlags | ComputeSizeFlags::eShrinkWrap);
diff --git a/layout/generic/ReflowInput.h b/layout/generic/ReflowInput.h
index 541102a41a6a..bad643875df7 100644
--- a/layout/generic/ReflowInput.h
+++ b/layout/generic/ReflowInput.h
@@ -220,6 +220,7 @@ public:
     bool mStaticPosIsCBOrigin : 1; // the STATIC_POS_IS_CB_ORIGIN ctor flag
     bool mIClampMarginBoxMinSize : 1; // the I_CLAMP_MARGIN_BOX_MIN_SIZE ctor flag
     bool mBClampMarginBoxMinSize : 1; // the B_CLAMP_MARGIN_BOX_MIN_SIZE ctor flag
+    bool mApplyAutoMinSize : 1;       // the I_APPLY_AUTO_MIN_SIZE ctor flag
 
     // If set, the following two flags indicate that:
     // (1) this frame is absolutely-positioned (or fixed-positioned).
@@ -739,6 +740,9 @@ public:
 
     // Pass ComputeSizeFlags::eBClampMarginBoxMinSize to ComputeSize().
     B_CLAMP_MARGIN_BOX_MIN_SIZE = (1<<6),
+
+    // Pass ComputeSizeFlags::eIApplyAutoMinSize to ComputeSize().
+    I_APPLY_AUTO_MIN_SIZE = (1<<7),
   };
 
   // This method initializes various data members. It is automatically
diff --git a/layout/generic/nsGridContainerFrame.cpp b/layout/generic/nsGridContainerFrame.cpp
index 3c85d09be1c2..38cada4b9ebd 100644
--- a/layout/generic/nsGridContainerFrame.cpp
+++ b/layout/generic/nsGridContainerFrame.cpp
@@ -5012,6 +5012,9 @@ nsGridContainerFrame::ReflowInFlowChild(nsIFrame*              aChild,
     } else {
       aChild->Properties().Delete(BClampMarginBoxMinSizeProperty());
     }
+    if ((aGridItemInfo->mState[childIAxis] & ItemState::eApplyAutoMinSize)) {
+      flags |= ReflowInput::I_APPLY_AUTO_MIN_SIZE;
+    }
   }
 
   if (!isConstrainedBSize) {
diff --git a/layout/generic/nsIFrame.h b/layout/generic/nsIFrame.h
index afdce684fa04..8b1e31f22248 100644
--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -2250,6 +2250,14 @@ public:
      */
     eIClampMarginBoxMinSize = 1 << 2, // clamp in our inline axis
     eBClampMarginBoxMinSize = 1 << 3, // clamp in our block axis
+    /**
+     * The frame is stretching (per CSS Box Alignment) and doesn't have an
+     * Automatic Minimum Size in the indicated axis.
+     * (may be used for both flex/grid items, but currently only used for Grid)
+     * https://drafts.csswg.org/css-grid/#min-size-auto
+     * https://drafts.csswg.org/css-align-3/#valdef-justify-self-stretch
+     */
+    eIApplyAutoMinSize = 1 << 4, // only has an effect when eShrinkWrap is false
   };
 
   /**