From a9ac8b390ff6e2d0c4daa5b872913db1b77851a7 Mon Sep 17 00:00:00 2001
From: Brian Smith <brian@dbsoft.org>
Date: Mon, 27 Jul 2026 12:36:24 -0500
Subject: [PATCH] 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 | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/layout/forms/nsHTMLButtonControlFrame.cpp b/layout/forms/nsHTMLButtonControlFrame.cpp
index d1fbfbabd7..794834e906 100644
--- a/layout/forms/nsHTMLButtonControlFrame.cpp
+++ b/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