From 8e58b5c8fb937459a1ec72887a2b3ccb1f91e185 Mon Sep 17 00:00:00 2001
From: Cameron Kaiser <classilla@floodgap.com>
Date: Sat, 6 May 2017 14:14:33 -0700
Subject: [PATCH] prerequisite for #375: M1347759

---
 layout/generic/nsAbsoluteContainingBlock.cpp | 50 +++++++++++++-------
 1 file changed, 33 insertions(+), 17 deletions(-)

diff --git a/layout/generic/nsAbsoluteContainingBlock.cpp b/layout/generic/nsAbsoluteContainingBlock.cpp
index 521a0cbc0..7c9d54d94 100644
--- a/layout/generic/nsAbsoluteContainingBlock.cpp
+++ b/layout/generic/nsAbsoluteContainingBlock.cpp
@@ -262,22 +262,6 @@ nsAbsoluteContainingBlock::FrameDependsOnContainer(nsIFrame* f,
         !IsFixedMarginSize(margin->mMargin.GetIEnd(wm))) {
       return true;
     }
-    if (!wm.IsBidiLTR()) {
-      // Note that even if 'istart' is a length, our position can
-      // still depend on the containing block isze, because if
-      // 'iend' is also a length we will discard 'istart' and be
-      // positioned relative to the containing block iend edge.
-      // 'istart' length and 'iend' auto is the only combination
-      // we can be sure of.
-      if (!IsFixedOffset(pos->mOffset.GetIStart(wm)) ||
-          pos->mOffset.GetIEndUnit(wm) != eStyleUnit_Auto) {
-        return true;
-      }
-    } else {
-      if (!IsFixedOffset(pos->mOffset.GetIStart(wm))) {
-        return true;
-      }
-    }
   }
   if (wm.IsVertical() ? aCBWidthChanged : aCBHeightChanged) {
     // See if f's block-size might have changed.
@@ -302,10 +286,42 @@ nsAbsoluteContainingBlock::FrameDependsOnContainer(nsIFrame* f,
         !IsFixedMarginSize(margin->mMargin.GetBEnd(wm))) {
       return true;
     }
-    if (!IsFixedOffset(pos->mOffset.GetBStart(wm))) {
+  }
+
+  // Since we store coordinates relative to top and left, the position
+  // of a frame depends on that of its container if it is fixed relative
+  // to the right or bottom, or if it is positioned using percentages
+  // relative to the left or top.  Because of the dependency on the
+  // sides (left and top) that we use to store coordinates, these tests
+  // are easier to do using physical coordinates rather than logical.
+  if (aCBWidthChanged) {
+    if (!IsFixedOffset(pos->mOffset.GetLeft())) {
+      return true;
+    }
+    // Note that even if 'left' is a length, our position can still
+    // depend on the containing block width, because if our direction or
+    // writing-mode moves from right to left (in either block or inline
+    // progression) and 'right' is not 'auto', we will discard 'left'
+    // and be positioned relative to the containing block right edge.
+    // 'left' length and 'right' auto is the only combination we can be
+    // sure of.
+    if ((wm.GetInlineDir() == WritingMode::eInlineRTL ||
+         wm.GetBlockDir() == WritingMode::eBlockRL) &&
+        pos->mOffset.GetRightUnit() != eStyleUnit_Auto) {
       return true;
     }
   }
+  if (aCBHeightChanged) {
+    if (!IsFixedOffset(pos->mOffset.GetTop())) {
+      return true;
+    }
+    // See comment above for width changes.
+    if (wm.GetInlineDir() == WritingMode::eInlineBTT &&
+        pos->mOffset.GetBottomUnit() != eStyleUnit_Auto) {
+      return true;
+    }
+  }
+
   return false;
 }