Fix page origin, stale fragment tree and anchor page index

- Keep the top margin on every PDF page
- Paint the whole document on the continuous surface when the container has a Y offset
- Reset the fragment tree in Clear()
- Pass one-based page numbers to AddDocumentLink
- Don't paginate descendants of fixed boxes

Author: eXpl0it3r

Source/HtmlRenderer.PdfSharp/PdfGenerator.cs

@@ -259,7 +259,7 @@ private static void HandleLinks(PdfDocument document, HtmlContainer container, X
                     var pageIndex = slotToPage[fragmentainer.SlotIndex];
 
                     // fucking position is from the bottom of the page
-                    var xRect = new XRect(link.Rectangle.Left, orgPageSize.Height - (link.Rectangle.Height + link.Rectangle.Top - bandTop), link.Rectangle.Width, link.Rectangle.Height);
+                    var xRect = new XRect(link.Rectangle.Left, orgPageSize.Height - (link.Rectangle.Height + link.Rectangle.Top - fragmentainer.LocalOriginY), link.Rectangle.Width, link.Rectangle.Height);
 
                     if (link.IsAnchor)
                     {
@@ -271,7 +271,7 @@ private static void HandleLinks(PdfDocument document, HtmlContainer container, X
                             // document links to the same page as the link is not allowed
                             if (anchorSlot.HasValue && slotToPage.TryGetValue(anchorSlot.Value, out var anchorPageIdx) && pageIndex != anchorPageIdx)
                             {
-                                document.Pages[pageIndex].AddDocumentLink(new PdfRectangle(xRect), anchorPageIdx);
+                                document.Pages[pageIndex].AddDocumentLink(new PdfRectangle(xRect), anchorPageIdx + 1); // PdfSharp page numbers are one-based
                             }
                         }
                     }

Source/HtmlRenderer/Core/Fragmentation/BlockFragmentation.cs

@@ -120,7 +120,7 @@ internal static bool TryGetForcedBreakTarget(CssBox box, CssBox prevSibling, dou
         internal static void RelocateIfNeeded(RGraphics g, CssBox child)
         {
             var container = child.HtmlContainer;
-            if (container == null || !container.HasRealPageGrid || child.IsOutOfFlow)
+            if (container == null || !container.HasRealPageGrid || child.IsOutOfFlow || child.IsFixed)
                 return;
 
             var top = child.EffectiveTop;
@@ -188,7 +188,7 @@ internal static void RelocateIfNeeded(RGraphics g, CssBox child)
         internal static void EnforceKeepWithNext(RGraphics g, CssBox child)
         {
             var container = child.HtmlContainer;
-            if (container == null || !container.HasRealPageGrid || child.IsOutOfFlow)
+            if (container == null || !container.HasRealPageGrid || child.IsOutOfFlow || child.IsFixed)
                 return;
 
             var prevSibling = DomUtils.GetPreviousSibling(child);

Source/HtmlRenderer/Core/Fragmentation/FragmentEmitter.cs

@@ -39,9 +39,10 @@ internal FragmentTree Finish()
             if (!_container.HasRealPageGrid)
             {
                 // No bounded page grid (WinForms/WPF's continuous-scroll convention, or any container
-                // that never set a real PageSize) - the whole document is one fragmentainer.
+                // that never set a real PageSize) - the whole document is one fragmentainer. The band is
+                // unbounded since content starts at the root's Location.Y, not at 0.
                 var rect = new RRect(RPoint.Empty, _container.ActualSize);
-                var band = new PageBand(0, rect.Height);
+                var band = new PageBand(0, double.MaxValue);
                 var geometry = new PageBandGeometry(0, rect.Height, _container.MarginTop, _container.MarginRight, _container.MarginBottom, _container.MarginLeft);
                 var rootFragment = BuildBoxFragment(root, 0, band);
                 var fragmentainer = new FragmentainerFragment(rect, 0, geometry, 0, rootFragment);
@@ -71,7 +72,7 @@ internal FragmentTree Finish()
             {
                 var bandTop = _container.PageTopOf(slot);
                 var bandBottom = _container.PageBottomOf(slot);
-                var band = new PageBand(bandTop, bandBottom);
+                var band = new PageBand(bandTop, bandBottom, bandTop - _container.MarginTop);
 
                 // CSS Paged Media 3 3.2: a page-slot no box has any content in is never materialized -
                 // this falls out of the walk rather than being special-cased, since a box only gets
@@ -94,7 +95,7 @@ internal FragmentTree Finish()
 
                 var rect = new RRect(0, 0, _container.PageSize.Width, band.Height);
                 var geometry = new PageBandGeometry(bandTop, band.Height, _container.MarginTop, _container.MarginRight, _container.MarginBottom, _container.MarginLeft);
-                fragmentainers.Add(new FragmentainerFragment(rect, slot, geometry, bandTop, rootFragment));
+                fragmentainers.Add(new FragmentainerFragment(rect, slot, geometry, band.Origin, rootFragment));
             }
 
             return new FragmentTree(fragmentainers);
@@ -259,7 +260,7 @@ private static RRect Clip(RRect rect, PageBand band)
             return new RRect(rect.X, top, rect.Width, Math.Max(0, bottom - top));
         }
 
-        private static RRect ToLocal(RRect rect, PageBand band) => new RRect(rect.X, rect.Y - band.Top, rect.Width, rect.Height);
+        private static RRect ToLocal(RRect rect, PageBand band) => new RRect(rect.X, rect.Y - band.Origin, rect.Width, rect.Height);
 
         /// <summary>
         /// A no-op <see cref="SliceGeometry"/> - every edge is treated as a real box edge, since real

Source/HtmlRenderer/Core/Fragmentation/InlineFragmentation.cs

@@ -47,7 +47,7 @@ internal static void ApplyLineBreaking(CssBox blockBox)
             // CollectFixedRoots) - unlike a float or an absolutely-positioned box, which stay in normal
             // document flow and must still paginate like anything else, the UA "must not paginate the
             // content of fixed-positioned boxes" (css-position-3), so this correction does not apply.
-            if (container == null || !container.HasRealPageGrid || blockBox.Position == CssConstants.Fixed)
+            if (container == null || !container.HasRealPageGrid || blockBox.IsFixed)
                 return;
 
             var lines = blockBox.LineBoxes;

Source/HtmlRenderer/Core/Fragmentation/PageBand.cs

@@ -10,13 +10,25 @@ namespace TheArtOfDev.HtmlRenderer.Core.Fragmentation
     internal readonly struct PageBand
     {
         public PageBand(double top, double bottom)
+            : this(top, bottom, top)
+        {
+        }
+
+        public PageBand(double top, double bottom, double origin)
         {
             Top = top;
             Bottom = bottom;
+            Origin = origin;
         }
 
         public double Top { get; }
         public double Bottom { get; }
+
+        /// <summary>
+        /// The document Y that maps to Y=0 on the fragmentainer's own surface. On a page this is the band's
+        /// top minus the page's top margin, so content keeps the margin above it.
+        /// </summary>
+        public double Origin { get; }
         public double Height => Bottom - Top;
 
         public bool Contains(double y) => y >= Top && y < Bottom;

Source/HtmlRenderer/Core/HtmlContainerInt.cs

@@ -654,6 +654,8 @@ public void Clear()
 
                 _hoverBoxes = null;
             }
+
+            FragmentTree = null;
         }
 
         /// <summary>
@@ -875,18 +877,9 @@ public void PerformPaint(RGraphics g)
         /// <param name="g">the device to use to render</param>
         /// <param name="fragmentainer">the fragmentainer to paint</param>
         /// <remarks>
-        /// The pushed clip's Y origin is always 0, never <see cref="MarginTop"/>/<see cref="_location"/>'s
-        /// Y - unlike <see cref="PerformPaint(RGraphics)"/>'s multi-fragmentainer loop (which paints every
-        /// band back onto one continuous, absolute-Y surface via a per-band page-origin translate),
-        /// <paramref name="fragmentainer"/> here is painted alone onto its own fresh surface (a real PDF
-        /// page, one per <see cref="PdfGenerator"/> loop iteration) with no such translate - so its content
-        /// paints at exactly the fragment-local coordinates <see cref="Fragmentation.FragmentEmitter"/>
-        /// already produced (band-local Y = document Y - band top, per that type's own doc comment). A real
-        /// bug found while confirming this: the clip previously started at Y=<see cref="MarginTop"/>
-        /// (mirroring the single-surface overload's own absolute-Y convention), silently clipping away the
-        /// first <see cref="MarginTop"/>-tall strip of every single page's own content - confirmed by a
-        /// list item landing entirely within that clipped strip and never appearing in the paint log at all,
-        /// with no exception raised (the visibility cull is a quiet no-op, not a thrown error).
+        /// The fragmentainer is painted alone on its own surface (one PDF page). Its local coordinates already
+        /// keep the page's top margin (see <see cref="Fragmentation.PageBand.Origin"/>), so the clip starts at
+        /// <see cref="MarginTop"/> like the continuous overload's does.
         /// </remarks>
         internal void PerformPaint(RGraphics g, Fragments.FragmentainerFragment fragmentainer)
         {
@@ -895,11 +888,11 @@ internal void PerformPaint(RGraphics g, Fragments.FragmentainerFragment fragment
 
             if (MaxSize.Height > 0)
             {
-                g.PushClip(new RRect(_location.X, 0, Math.Min(_maxSize.Width, PageSize.Width), Math.Min(_maxSize.Height, PageSize.Height)));
+                g.PushClip(new RRect(_location.X, _location.Y, Math.Min(_maxSize.Width, PageSize.Width), Math.Min(_maxSize.Height, PageSize.Height)));
             }
             else
             {
-                g.PushClip(new RRect(MarginLeft, 0, PageSize.Width, PageSize.Height));
+                g.PushClip(new RRect(MarginLeft, MarginTop, PageSize.Width, PageSize.Height));
             }
 
             new Paint.FragmentPainter(this).Paint(g, fragmentainer);

Source/Test/HtmlRenderer.IntegrationTest/Painting/PageOriginPaintIntegrationTests.cs

@@ -0,0 +1,67 @@
+using System;
+using System.Linq;
+using HtmlRenderer.IntegrationTest.TestSupport;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using TheArtOfDev.HtmlRenderer.Adapters.Entities;
+using TheArtOfDev.HtmlRenderer.Core;
+
+namespace HtmlRenderer.IntegrationTest.Painting;
+
+/// <summary>
+/// Where fragment content lands on the painted surface: a PDF page keeps its top margin, and the continuous
+/// (non-paged) surface paints everything even when the container doesn't start at Y=0.
+/// </summary>
+[TestClass]
+public sealed class PageOriginPaintIntegrationTests
+{
+    [TestMethod]
+    public void EveryPage_KeepsTheTopMargin()
+    {
+        const double margin = 20;
+        var (_, container) = PaintHarness.LayoutPaginated(PaintHarness.Wrap(
+            "<p style='margin:0;height:150px'>PageOneMarker</p>"
+            + "<p style='margin:0;height:150px;page-break-before:always'>PageTwoMarker</p>"),
+            pageHeight: 200, margin: margin);
+
+        Assert.AreEqual(2, container.FragmentTree!.Fragmentainers.Count);
+
+        var first = PaintHarness.PaintPage(container, 0).DrawStringCalls.Single(c => c.Text.Contains("PageOneMarker"));
+        var second = PaintHarness.PaintPage(container, 1).DrawStringCalls.Single(c => c.Text.Contains("PageTwoMarker"));
+
+        Assert.IsGreaterThanOrEqualTo(margin - 0.001, first.Point.Y, "first page content starts above the top margin");
+        Assert.AreEqual(first.Point.Y, second.Point.Y, 0.001, "both pages start their content at the same Y");
+    }
+
+    [TestMethod]
+    public void ContinuousSurface_WithAnOffset_StillPaintsTheBottomOfTheDocument()
+    {
+        var container = new HtmlContainerInt(new MockAdapter())
+        {
+            MaxSize = new RSize(400, 0),
+            Location = new RPoint(0, 300),
+            PageSize = new RSize(99999, 99999)
+        };
+        container.SetHtml(PaintHarness.Wrap(
+            "<div style='margin:0;height:400px'>top</div><p style='margin:0'>BottomMarker</p>"));
+
+        using var layout = new RecordingGraphics();
+        container.PerformLayout(layout);
+        Assert.IsFalse(container.HasRealPageGrid);
+
+        var g = new RecordingGraphics();
+        container.PerformPaint(g);
+
+        Assert.IsTrue(g.DrawStringCalls.Any(c => c.Text.Contains("BottomMarker")));
+    }
+
+    [TestMethod]
+    public void Clear_DropsTheFragmentTree()
+    {
+        var (_, container) = PaintHarness.Layout(PaintHarness.Wrap("<p>x</p>"));
+        Assert.IsNotNull(container.FragmentTree);
+
+        container.Clear();
+
+        Assert.IsNull(container.FragmentTree);
+    }
+}