Bug 1591925 - When resolving min-size:auto on the main axis of flex items, treat `overflow:-moz-hidden-unscrollable` the same as `overflow:visible`. r=dholbert

Differential Revision: https://phabricator.services.mozilla.com/D52401

--HG--
extra : moz-landing-system : lando

layout/generic/nsFlexContainerFrame.cpp

@@ -2093,18 +2093,19 @@ void FlexItem::CheckForMinSizeAuto(const ReflowInput& aFlexItemReflowInput,
   // the flex container's main axis) iff:
   // (a) its computed value is "auto"
   // (b) the "overflow" sub-property in the same axis (the main axis) has a
-  //     computed value of "visible"
+  //     computed value of "visible" and the item does not create a scroll
+  //     container.
   const auto& mainMinSize = aAxisTracker.IsRowOriented()
                                 ? pos->MinISize(aAxisTracker.GetWritingMode())
                                 : pos->MinBSize(aAxisTracker.GetWritingMode());
 
-  // NOTE: Technically we should be checking the 'overflow' subproperty in the
-  // main axis. But since we only care whether it's 'visible', we can check
-  // either subproperty -- because they must be BOTH 'visible' or BOTH
-  // non-'visible' due to the way the subproperties interact.
+  // If the scrollable overflow makes us create a scroll container, then we
+  // don't need to do any extra resolution for our `min-size:auto` value.
+  // We don't need to check for scrollable overflow in a particular axis
+  // because this will be true for both or neither axis.
   mNeedsMinSizeAutoResolution =
       IsAutoOrEnumOnBSize(mainMinSize, IsInlineAxisMainAxis()) &&
-      disp->mOverflowX == StyleOverflow::Visible;
+      !disp->IsScrollableOverflow();
 }
 
 nscoord FlexItem::GetBaselineOffsetFromOuterCrossEdge(