--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -1559,42 +1559,41 @@ ReflowInput::InitAbsoluteConstraints(nsP
   bool bStartIsAuto = styleOffset.GetBStartUnit(cbwm) == eStyleUnit_Auto;
   bool bEndIsAuto   = styleOffset.GetBEndUnit(cbwm) == eStyleUnit_Auto;
 
   // If both 'left' and 'right' are 'auto' or both 'top' and 'bottom' are
   // 'auto', then compute the hypothetical box position where the element would
   // have been if it had been in the flow
   nsHypotheticalPosition hypotheticalPos;
   if ((iStartIsAuto && iEndIsAuto) || (bStartIsAuto && bEndIsAuto)) {
+    nsIFrame* placeholderFrame =
+      aPresContext->PresShell()->GetPlaceholderFrameFor(mFrame);
+    NS_ASSERTION(placeholderFrame, "no placeholder frame");
+
+    if (placeholderFrame->HasAnyStateBits(
+          PLACEHOLDER_STATICPOS_NEEDS_CSSALIGN)) {
+      DebugOnly<nsIFrame*> placeholderParent = placeholderFrame->GetParent();
+      MOZ_ASSERT(placeholderParent, "shouldn't have unparented placeholders");
+      MOZ_ASSERT(placeholderParent->IsFlexOrGridContainer(),
+                 "This flag should only be set on grid/flex children");
+
+      // If the (as-yet unknown) static position will determine the inline
+      // and/or block offsets, set flags to note those offsets aren't valid
+      // until we can do CSS Box Alignment on the OOF frame.
+      mFlags.mIOffsetsNeedCSSAlign = (iStartIsAuto && iEndIsAuto);
+      mFlags.mBOffsetsNeedCSSAlign = (bStartIsAuto && bEndIsAuto);
+    }
+
     if (mFlags.mStaticPosIsCBOrigin) {
-      // XXXdholbert This whole clause should be removed in bug 1269017, where
-      // we'll be making abpsos grid children share our CSS Box Alignment code.
       hypotheticalPos.mWritingMode = cbwm;
       hypotheticalPos.mIStart = nscoord(0);
       hypotheticalPos.mBStart = nscoord(0);
     } else {
-      nsIFrame* placeholderFrame =
-        aPresContext->PresShell()->GetPlaceholderFrameFor(mFrame);
-      NS_ASSERTION(placeholderFrame, "no placeholder frame");
       CalculateHypotheticalPosition(aPresContext, placeholderFrame, cbrs,
                                     hypotheticalPos, aFrameType);
-
-      if (placeholderFrame->HasAnyStateBits(
-            PLACEHOLDER_STATICPOS_NEEDS_CSSALIGN)) {
-        DebugOnly<nsIFrame*> placeholderParent = placeholderFrame->GetParent();
-        MOZ_ASSERT(placeholderParent, "shouldn't have unparented placeholders");
-        MOZ_ASSERT(placeholderParent->IsFlexOrGridContainer(),
-                   "This flag should only be set on grid/flex children");
-
-        // If the (as-yet unknown) static position will determine the inline
-        // and/or block offsets, set flags to note those offsets aren't valid
-        // until we can do CSS Box Alignment on the OOF frame.
-        mFlags.mIOffsetsNeedCSSAlign = (iStartIsAuto && iEndIsAuto);
-        mFlags.mBOffsetsNeedCSSAlign = (bStartIsAuto && bEndIsAuto);
-      }
     }
   }
 
   // Initialize the 'left' and 'right' computed offsets
   // XXX Handle new 'static-position' value...
 
   // Size of the containing block in its writing mode
   LogicalSize cbSize = aCBSize;

--- a/layout/generic/ReflowInput.h
+++ b/layout/generic/ReflowInput.h
@@ -229,18 +229,16 @@ public:
     // When these bits are set, the offset values (IStart/IEnd, BStart/BEnd)
     // represent the "start" edge of the frame's CSS Box Alignment container
     // area, in that axis -- and these offsets need to be further-resolved
     // (with CSS Box Alignment) after we know the OOF frame's size.
     // NOTE: The "I" and "B" (for "Inline" and "Block") refer the axes of the
     // *containing block's writing-mode*, NOT mFrame's own writing-mode. This
     // is purely for convenience, since that's the writing-mode we're dealing
     // with when we set & react to these bits.
-    // XXXdholbert These new bits will probably make the "mStaticPosIsCBOrigin"
-    // bit obsolete -- consider removing it in bug 1269017.
     uint32_t mIOffsetsNeedCSSAlign:1;
     uint32_t mBOffsetsNeedCSSAlign:1;
   };
 
 #ifdef DEBUG
   // Reflow trace methods.  Defined in nsFrame.cpp so they have access
   // to the display-reflow infrastructure.
   static void* DisplayInitOffsetsEnter(
@@ -724,18 +722,20 @@ public:
     // The caller wants shrink-wrap behavior (i.e. ComputeSizeFlags::eShrinkWrap
     // will be passed to ComputeSize()).
     COMPUTE_SIZE_SHRINK_WRAP = (1<<2),
 
     // The caller wants 'auto' bsize behavior (ComputeSizeFlags::eUseAutoBSize
     // will be be passed to ComputeSize()).
     COMPUTE_SIZE_USE_AUTO_BSIZE = (1<<3),
 
-    // The caller wants the abs.pos. static-position resolved at the origin
-    // of the containing block, i.e. at LogicalPoint(0, 0).
+    // The caller wants the abs.pos. static-position resolved at the origin of
+    // the containing block, i.e. at LogicalPoint(0, 0). (Note that this
+    // doesn't necessarily mean that (0, 0) is the *correct* static position
+    // for the frame in question.)
     STATIC_POS_IS_CB_ORIGIN = (1<<4),
 
     // Pass ComputeSizeFlags::eIClampMarginBoxMinSize to ComputeSize().
     I_CLAMP_MARGIN_BOX_MIN_SIZE = (1<<5),
 
     // Pass ComputeSizeFlags::eBClampMarginBoxMinSize to ComputeSize().
     B_CLAMP_MARGIN_BOX_MIN_SIZE = (1<<6),
   };

--- a/layout/generic/nsAbsoluteContainingBlock.cpp
+++ b/layout/generic/nsAbsoluteContainingBlock.cpp
@@ -629,18 +629,21 @@ nsAbsoluteContainingBlock::ReflowAbsolut
                  "Must have a useful inline-size _somewhere_");
     availISize =
       aReflowInput.ComputedSizeWithPadding(wm).ISize(wm);
   }
 
   uint32_t rsFlags = 0;
   if (aFlags & AbsPosReflowFlags::eIsGridContainerCB) {
     // When a grid container generates the abs.pos. CB for a *child* then
-    // the static-position is the CB origin (i.e. of the grid area rect).
-    // https://drafts.csswg.org/css-grid/#static-position
+    // the static position is determined via CSS Box Alignment within the
+    // abs.pos. CB (a grid area, i.e. a piece of the grid). In this scenario,
+    // due to the multiple coordinate spaces in play, we use a convenience flag
+    // to simply have the child's ReflowInput give it a static position at its
+    // abs.pos. CB origin, and then we'll align & offset it from there.
     nsIFrame* placeholder =
       aPresContext->PresShell()->GetPlaceholderFrameFor(aKidFrame);
     if (placeholder && placeholder->GetParent() == aDelegatingFrame) {
       rsFlags |= ReflowInput::STATIC_POS_IS_CB_ORIGIN;
     }
   }
   ReflowInput kidReflowInput(aPresContext, aReflowInput, aKidFrame,
                                    LogicalSize(wm, availISize,