Issue #1370 - Part 3: Implement `content` keyword for `flex-basis` property

Partially based on https://bugzilla.mozilla.org/show_bug.cgi?id=1105111

2 files changed, 37 insertions, 4 deletions

diff --git a/layout/generic/nsContainerFrame.cpp b/layout/generic/nsContainerFrame.cpp
index 125d602635..dd20ccffee 100644
--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -944,8 +944,29 @@ nsContainerFrame::ComputeAutoSize(nsRenderingContext* aRenderingContext,
                        aBorder.ISize(aWM) - aPadding.ISize(aWM);
   // replaced elements always shrink-wrap
   if ((aFlags & ComputeSizeFlags::eShrinkWrap) || IsFrameOfType(eReplaced)) {
-    // don't bother setting it if the result won't be used
-    if (StylePosition()->ISize(aWM).GetUnit() == eStyleUnit_Auto) {
+    const nsStylePosition* position = StylePosition();
+
+    bool isFlexBasisContent = false;
+    if (IsFlexItem()) {
+      uint32_t flexDirection =
+        GetParent()->StylePosition()->mFlexDirection;
+      bool isInlineFlexItem =
+        flexDirection == NS_STYLE_FLEX_DIRECTION_ROW ||
+        flexDirection == NS_STYLE_FLEX_DIRECTION_ROW_REVERSE;
+      isFlexBasisContent =
+        position->mFlexBasis.GetUnit() == eStyleUnit_Enumerated &&
+        position->mFlexBasis.GetIntValue() == NS_STYLE_FLEX_BASIS_CONTENT &&
+        isInlineFlexItem;
+    }
+
+    // Only bother computing our 'auto' ISize if the result will be used.
+    // It'll be used under two scenarios:
+    // - If our ISize property is itself 'auto'.
+    // - If we're using flex-basis in place of our ISize property (i.e. we're a
+    // flex item with our inline axis being the main axis), AND we have
+    // flex-basis:content.
+    bool isAuto = position->ISize(aWM).GetUnit() == eStyleUnit_Auto;
+    if (isAuto || isFlexBasisContent) {
       result.ISize(aWM) = ShrinkWidthToFit(aRenderingContext, availBased, aFlags);
     }
   } else {
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 99442e681f..141c148673 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -4647,11 +4647,23 @@ nsFrame::SetCoordToFlexBasis(bool aIsInlineFlexItem,
     return;
   }
 
+  const nsStyleCoord* newCoord = aFlexBasis;
+
+  // Having 'content' as the value of the 'flex-basis' property is
+  // equivalent to setting both 'flex-basis' and the main size
+  // properties to 'auto', which is why a dummy 'auto' value will
+  // be used here for the main size property.
+  if (aFlexBasis->GetUnit() == eStyleUnit_Enumerated &&
+      aFlexBasis->GetIntValue() == NS_STYLE_FLEX_BASIS_CONTENT) {
+    static const nsStyleCoord autoStyleCoord(eStyleUnit_Auto);
+    newCoord = &autoStyleCoord;
+  }
+
   // Override whichever styleCoord is in the flex container's main axis
   if (aIsInlineFlexItem) {
-    *aInlineStyle = aFlexBasis;
+    *aInlineStyle = newCoord;
   } else {
-    *aBlockStyle = aFlexBasis;
+    *aBlockStyle = newCoord;
   }
 }