Issue #3153 - Make sure buttons report a definite size for flex containers.

Second try, this time ONLY do it for flex container or grids.
This should prevent layout problems in non-flex scenarios.

layout/forms/nsHTMLButtonControlFrame.cpp

@@ -234,7 +234,18 @@ nsHTMLButtonControlFrame::ReflowButtonContents(nsPresContext* aPresContext,
 {
   WritingMode wm = GetWritingMode();
   LogicalSize availSize = aButtonReflowInput.ComputedSize(wm);
-  availSize.BSize(wm) = NS_INTRINSICSIZE;
+
+  // Identify if the inner anonymous block is actually functioning as a flex or grid container.
+  nsIAtom* kidType = aFirstKid->GetType();
+  bool isFlexOrGrid = (kidType == nsGkAtoms::flexContainerFrame ||
+                       kidType == nsGkAtoms::gridContainerFrame);
+
+  // Preserve the button's definite BSize ONLY for flex/grid children so they
+  // see a definite containing block main size. For standard buttons, clobber
+  // to intrinsic so standard vertical centering (extraSpace) calculations work.
+  if (!isFlexOrGrid || aButtonReflowInput.ComputedBSize() == NS_INTRINSICSIZE) {
+    availSize.BSize(wm) = NS_INTRINSICSIZE;
+  }
 
   // shorthand for a value we need to use in a bunch of places
   const LogicalMargin& clbp = aButtonReflowInput.ComputedLogicalBorderPadding();
@@ -248,6 +259,12 @@ nsHTMLButtonControlFrame::ReflowButtonContents(nsPresContext* aPresContext,
                                   aFirstKid,
                                   availSize);
 
+  // Apply the definite BSize directly on the inner reflow input ONLY if it's
+  // a flex or grid container, allowing it to be used as-is.
+  if (isFlexOrGrid && aButtonReflowInput.ComputedBSize() != NS_INTRINSICSIZE) {
+    contentsReflowInput.SetComputedBSize(aButtonReflowInput.ComputedBSize());
+  }
+
   nsReflowStatus contentsReflowStatus;
   ReflowOutput contentsDesiredSize(aButtonReflowInput);
   childPos.B(wm) = 0; // This will be set properly later, after reflowing the