Bug 1804794 - Do not allow CSS named page breaks except in specific subtrees. r=dholbert

This currently only includes block frames, grid containers, and flex
containers, and the document and pagination frames. It is possible more frames
will need to be added or more advanced checks in the future.

This adds some related tests to ignoring some subtrees, but are expected fails
until bug 1816570 is fixed.

Differential Revision: https://phabricator.services.mozilla.com/D169018

layout/generic/ReflowInput.cpp

@@ -140,6 +140,9 @@ ReflowInput::ReflowInput(nsPresContext* aPresContext, nsIFrame* aFrame,
   if (!aFlags.contains(InitFlag::CallerWillInit)) {
     Init(aPresContext);
   }
+  // If we encounter a PageContent frame, this will be flipped on and the pref
+  // layout.css.named-pages.enabled will be checked.
+  mFlags.mCanHaveClassABreakpoints = false;
 }
 
 // Initialize a reflow input for a child frame's reflow. Some state
@@ -199,6 +202,49 @@ ReflowInput::ReflowInput(nsPresContext* aPresContext,
   mFlags.mStaticPosIsCBOrigin = aFlags.contains(InitFlag::StaticPosIsCBOrigin);
   mFlags.mIOffsetsNeedCSSAlign = mFlags.mBOffsetsNeedCSSAlign = false;
 
+  // aPresContext->IsPaginated() and the named pages pref should have been
+  // checked when constructing the root ReflowInput.
+  if (aParentReflowInput.mFlags.mCanHaveClassABreakpoints) {
+    MOZ_ASSERT(aPresContext->IsPaginated(),
+               "mCanHaveClassABreakpoints set during non-paginated reflow.");
+    MOZ_ASSERT(StaticPrefs::layout_css_named_pages_enabled(),
+               "mCanHaveClassABreakpoints should not be set when "
+               "layout.css.named_pages.enabled is false");
+  }
+
+  {
+    using mozilla::LayoutFrameType;
+    switch (mFrame->Type()) {
+      case LayoutFrameType::PageContent:
+        // PageContent requires paginated reflow.
+        MOZ_ASSERT(aPresContext->IsPaginated(),
+                   "nsPageContentFrame should not be in non-paginated reflow");
+        MOZ_ASSERT(!mFlags.mCanHaveClassABreakpoints,
+                   "mFlags.mCanHaveClassABreakpoints should have been "
+                   "initalized to false before we found nsPageContentFrame");
+        mFlags.mCanHaveClassABreakpoints =
+            StaticPrefs::layout_css_named_pages_enabled();
+        break;
+      case LayoutFrameType::Block:          // FALLTHROUGH
+      case LayoutFrameType::Canvas:         // FALLTHROUGH
+      case LayoutFrameType::FlexContainer:  // FALLTHROUGH
+      case LayoutFrameType::GridContainer:
+        // This frame type can have class A breakpoints, inherit this flag
+        // from the parent (this is done for all flags during construction).
+        // This also includes Canvas frames, as each PageContent frame always
+        // has exactly one child which is a Canvas frame.
+        // Do NOT include the subclasses of BlockFrame here, as the ones for
+        // which this could be applicable (ColumnSetWrapper and the MathML
+        // frames) cannot have class A breakpoints.
+        MOZ_ASSERT(mFlags.mCanHaveClassABreakpoints ==
+                   aParentReflowInput.mFlags.mCanHaveClassABreakpoints);
+        break;
+      default:
+        mFlags.mCanHaveClassABreakpoints = false;
+        break;
+    }
+  }
+
   if (aFlags.contains(InitFlag::DummyParentReflowInput) ||
       (mParentReflowInput->mFlags.mDummyParentReflowInput &&
        mFrame->IsTableFrame())) {

layout/generic/ReflowInput.h

@@ -562,6 +562,10 @@ struct ReflowInput : public SizeComputationInput {
     // children's block-size (after reflowing them).
     // https://drafts.csswg.org/css-sizing-4/#aspect-ratio-minimum
     bool mIsBSizeSetByAspectRatio : 1;
+
+    // If true, then children of this frame can generate class A breakpoints
+    // for paginated reflow.
+    bool mCanHaveClassABreakpoints : 1;
   };
   Flags mFlags;
 

layout/generic/nsBlockFrame.cpp

@@ -2647,11 +2647,23 @@ void nsBlockFrame::ReflowDirtyLines(BlockReflowState& aState) {
   // See: https://www.w3.org/TR/css-break-3/#btw-blocks
   const nsPresContext* const presCtx = aState.mPresContext;
   const bool canBreakForPageNames =
+      aState.mReflowInput.mFlags.mCanHaveClassABreakpoints &&
       aState.mReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE &&
-      presCtx->IsPaginated() && StaticPrefs::layout_css_named_pages_enabled() &&
       presCtx->GetPresShell()->GetRootFrame()->GetWritingMode().IsVertical() ==
           GetWritingMode().IsVertical();
 
+  // ReflowInput.mFlags.mCanHaveClassABreakpoints should respect the named
+  // pages pref and presCtx->IsPaginated, so we did not explicitly check these
+  // above when setting canBreakForPageNames.
+  if (canBreakForPageNames) {
+    MOZ_ASSERT(presCtx->IsPaginated(),
+               "canBreakForPageNames should not be set during non-paginated "
+               "reflow");
+    MOZ_ASSERT(StaticPrefs::layout_css_named_pages_enabled(),
+               "canBreakForPageNames should not be set when "
+               "layout.css.named_pages.enabled is false");
+  }
+
   // Reflow the lines that are already ours
   for (; line != line_end; ++line, aState.AdvanceToNextLine()) {
     DumpLine(aState, line, deltaBCoord, 0);

layout/printing/crashtests/1804794.html

@@ -0,0 +1,10 @@
+<script>
+window.onload = () => {
+  SpecialPowers.wrap(window).printPreview()?.close()
+}
+</script>
+<style>
+html { column-count: 5 }
+</style>
+<hr style="page: a">
+<a>A</a>

layout/printing/crashtests/crashtests.list

@@ -9,3 +9,4 @@ load 1748277.html # Bug 1751260
 load 1758199-1.html
 pref(layout.css.named-pages.enabled,true) load 1804571.html
 pref(layout.css.named-pages.enabled,true) load 1804798.html
+pref(layout.css.named-pages.enabled,true) load 1804794.html

layout/reftests/css-page/page-name-abspos-001-ref.html

@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div>a</div>
+    <div style="position: absolute; right: 0; top: 0;">b</div>
+  </body>
+</html>

layout/reftests/css-page/page-name-abspos-001.html

@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div>a</div>
+    <div style="page: b; position: absolute; right: 0; top: 0;">b</div>
+  </body>
+</html>

layout/reftests/css-page/page-name-abspos-002-ref.html

@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div style="position: absolute; left: 0; top: 0;">
+      <div>a</div>
+      <div>b</div>
+    </div>
+  </body>
+</html>

layout/reftests/css-page/page-name-abspos-002.html

@@ -0,0 +1,9 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div style="position: absolute; left: 0; top: 0;">
+      <div style="page: a">a</div>
+      <div style="page: b">b</div>
+    </div>
+  </body>
+</html>

layout/reftests/css-page/page-name-abspos-003-ref.html

@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div style="page: a">a</div>
+    <div style="page: b; position: absolute; left: 0; top: 0;">b</div>
+    <div style="page: c; break-before: page;">c</div>
+  </body>
+</html>

layout/reftests/css-page/page-name-abspos-003.html

@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div style="page: a">a</div>
+    <div style="page: b; position: absolute; left: 0; top: 0;">b</div>
+    <div style="page: c">c</div>
+  </body>
+</html>

layout/reftests/css-page/page-name-float-001-ref.html

@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div style="float: left;">a</div>
+    <div>b</div>
+  </body>
+</html>

layout/reftests/css-page/page-name-float-001.html

@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div style="page: a; float: left;">a</div>
+    <div style="page: b">b</div>
+  </body>
+</html>

layout/reftests/css-page/page-name-float-002-ref.html

@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div>a</div>
+    <div style="float: left;">b</div>
+    <div style="break-before: page;">c</div>
+  </body>
+</html>

layout/reftests/css-page/page-name-float-002.html

@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+<html class="reftest-paged">
+  <body>
+    <div style="page: a">a</div>
+    <div style="page: b; float: left;">b</div>
+    <div style="page: c">c</div>
+  </body>
+</html>

layout/reftests/css-page/reftest.list

@@ -1,11 +1,13 @@
 defaults pref(layout.css.named-pages.enabled,true)
 
-# https://bugzilla.mozilla.org/1764437
-== page-name-display-none-child.html page-name-display-none-child-ref.html
+== page-name-abspos-001.html page-name-abspos-001-ref.html
+fails == page-name-abspos-002.html page-name-abspos-002-ref.html # bug 1816570
+fails == page-name-abspos-003.html page-name-abspos-003-ref.html # bug 1816570
 == page-name-canvas-001.html page-name-canvas-001-ref.html
 == page-name-canvas-002.html page-name-canvas-002-ref.html
 == page-name-canvas-003.html page-name-canvas-003-ref.html
 == page-name-canvas-004.html page-name-canvas-004-ref.html
+== page-name-display-none-child.html page-name-display-none-child-ref.html
 == page-name-fixed-pos-001.html page-name-fixed-pos-001-ref.html
 == page-name-flex-001.html page-name-flex-001-ref.html
 == page-name-flex-002.html page-name-flex-002-ref.html
@@ -13,6 +15,8 @@ defaults pref(layout.css.named-pages.enabled,true)
 # This is affected by https://bugzilla.mozilla.org/1663079, however we can
 # still expect this to behave the same with break-after and page name breaks.
 == page-name-flex-004.html page-name-flex-004-ref.html
+== page-name-float-001.html page-name-float-001-ref.html
+fails == page-name-float-002.html page-name-float-002-ref.html # bug 1816570
 == page-name-img-001.html page-name-img-001-ref.html
 == page-name-img-002.html page-name-img-002-ref.html
 == page-name-img-003.html page-name-img-003-ref.html