From 3285bbf67ee9a03b95ba37919bbb22a3a31c273f Mon Sep 17 00:00:00 2001
From: Ting-Yu Lin <tlin@mozilla.com>
Date: Mon, 15 Nov 2021 18:51:29 +0000
Subject: [PATCH] Bug 1175509 Part 2 - Make TryLayout correctly consider
 ScrolledContentDependsOnBSize in vertical writing-mode. r=emilio

This patch potentially changes the behavior when the scrolled frame is in
vertical writing-mode, but it should be the correct way to handle the scrollbar
change.

Differential Revision: https://phabricator.services.mozilla.com/D130711
---
 layout/generic/nsGfxScrollFrame.cpp | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index e265f4886cac..2e99e9b8d061 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -437,10 +437,23 @@ bool nsHTMLScrollFrame::TryLayout(ScrollReflowInput* aState,
     return false;
   }
 
-  if (aAssumeVScroll != aState->mReflowedContentsWithVScrollbar ||
-      (aAssumeHScroll != aState->mReflowedContentsWithHScrollbar &&
-       ScrolledContentDependsOnBSize(aState))) {
-    if (aAssumeHScroll != aState->mReflowedContentsWithHScrollbar) {
+  const bool assumeVScrollChanged =
+      aAssumeVScroll != aState->mReflowedContentsWithVScrollbar;
+  const bool assumeHScrollChanged =
+      aAssumeHScroll != aState->mReflowedContentsWithHScrollbar;
+  const bool isVertical = GetWritingMode().IsVertical();
+
+  const bool shouldReflowScolledFrame = [=]() {
+    if (isVertical) {
+      return assumeHScrollChanged ||
+             (assumeVScrollChanged && ScrolledContentDependsOnBSize(aState));
+    }
+    return assumeVScrollChanged ||
+           (assumeHScrollChanged && ScrolledContentDependsOnBSize(aState));
+  }();
+
+  if (shouldReflowScolledFrame) {
+    if (isVertical ? assumeVScrollChanged : assumeHScrollChanged) {
       nsLayoutUtils::MarkIntrinsicISizesDirtyIfDependentOnBSize(
           mHelper.mScrolledFrame);
     }
@@ -644,9 +657,6 @@ bool nsHTMLScrollFrame::TryLayout(ScrollReflowInput* aState,
   return true;
 }
 
-// XXX Height/BSize mismatch needs to be addressed here; check the caller!
-// Currently this will only behave as expected for horizontal writing modes.
-// (See bug 1175509.)
 bool nsHTMLScrollFrame::ScrolledContentDependsOnBSize(
     ScrollReflowInput* aState) const {
   return mHelper.mScrolledFrame->HasAnyStateBits(