diff --git a/third_party/blink/renderer/core/layout/svg/layout_svg_image.cc b/third_party/blink/renderer/core/layout/svg/layout_svg_image.cc
index d502d06679e6a..f2d0e38d372c4 100644
--- a/third_party/blink/renderer/core/layout/svg/layout_svg_image.cc
+++ b/third_party/blink/renderer/core/layout/svg/layout_svg_image.cc
@@ -25,7 +25,9 @@
 
 #include "third_party/blink/renderer/core/layout/svg/layout_svg_image.h"
 
+#include "base/logging.h"
 #include "third_party/blink/renderer/core/layout/hit_test_location.h"
+#include "third_party/blink/renderer/core/page/page.h"
 #include "third_party/blink/renderer/core/layout/hit_test_result.h"
 #include "third_party/blink/renderer/core/layout/layout_image_resource.h"
 #include "third_party/blink/renderer/core/layout/layout_object_inlines.h"
@@ -249,6 +251,15 @@ bool LayoutSVGImage::NodeAtPoint(HitTestResult& result,
 
 void LayoutSVGImage::ImageChanged(WrappedImagePtr, CanDeferInvalidation defer) {
   NOT_DESTROYED();
+  if (!GetDocument().GetPage() ||
+      !GetDocument().GetPage()->IsOrdinary()) {
+    LOG(ERROR) << "[POC] LayoutSVGImage::ImageChanged in isolated doc "
+               << "url=" << GetDocument().Url().GetString().Utf8()
+               << " new_size=" << CalculateObjectSize().ToString()
+               << " old_size=" << object_bounding_box_.size().ToString()
+               << " will_SetNeedsLayout="
+               << (CalculateObjectSize() != object_bounding_box_.size());
+  }
   // Notify parent resources that we've changed. This also invalidates
   // references from resources (filters) that may have a cached
   // representation of this image/layout object.
diff --git a/third_party/blink/renderer/core/layout/svg/layout_svg_resource_pattern.cc b/third_party/blink/renderer/core/layout/svg/layout_svg_resource_pattern.cc
index 85e5ccb1d0494..cfdfd4328429f 100644
--- a/third_party/blink/renderer/core/layout/svg/layout_svg_resource_pattern.cc
+++ b/third_party/blink/renderer/core/layout/svg/layout_svg_resource_pattern.cc
@@ -23,6 +23,8 @@
 
 #include <memory>
 
+#include "base/debug/dump_without_crashing.h"
+#include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "third_party/blink/renderer/core/display_lock/display_lock_utilities.h"
 #include "third_party/blink/renderer/core/layout/svg/svg_layout_support.h"
@@ -235,6 +237,13 @@ PaintRecord LayoutSVGResourcePattern::AsPaintRecord(
   const auto* pattern_layout_object = To<LayoutSVGResourceContainer>(
       pattern_content_element->GetLayoutObject());
   DCHECK(pattern_layout_object);
+  // [POC] Diagnostic: log if the pattern subtree is dirty at paint time.
+  if (pattern_layout_object->NeedsLayout()) {
+    LOG(ERROR) << "[POC][HIT] LayoutSVGResourcePattern::AsPaintRecord: pattern "
+               << "subtree NeedsLayout()! obj=" << pattern_layout_object
+               << " in doc=" << &pattern_layout_object->GetDocument();
+    base::debug::DumpWithoutCrashing();
+  }
   DCHECK(!pattern_layout_object->NeedsLayout());
 
   SubtreeContentTransformScope content_transform_scope(tile_transform);
diff --git a/third_party/blink/renderer/core/paint/clip_path_clipper.cc b/third_party/blink/renderer/core/paint/clip_path_clipper.cc
index bd3bc38351daa..777152d1c7a0f 100644
--- a/third_party/blink/renderer/core/paint/clip_path_clipper.cc
+++ b/third_party/blink/renderer/core/paint/clip_path_clipper.cc
@@ -37,6 +37,8 @@
 #include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 #include "third_party/blink/renderer/platform/transforms/affine_transform.h"
 #include "third_party/skia/include/pathops/SkPathOps.h"
+#include "base/debug/dump_without_crashing.h"
+#include "base/logging.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 #include "ui/gfx/geometry/vector2d_f.h"
 
@@ -76,6 +78,14 @@ LayoutSVGResourceClipper* ResolveElementReference(
   if (DisplayLockUtilities::LockedAncestorPreventingLayout(*resource_clipper))
     return nullptr;
 
+  // [POC] Diagnostic: SECURITY_DCHECK is debug-only and only checks the
+  // container itself, not the subtree. Log if the subtree is dirty.
+  if (resource_clipper->NeedsLayout()) {
+    LOG(ERROR) << "[POC][HIT] ResolveElementReference: clip-path subtree "
+               << "NeedsLayout()! clipper=" << resource_clipper << " in doc="
+               << &resource_clipper->GetDocument();
+    base::debug::DumpWithoutCrashing();
+  }
   SECURITY_DCHECK(!resource_clipper->SelfNeedsFullLayout());
   return resource_clipper;
 }
diff --git a/third_party/blink/renderer/core/paint/svg_shape_painter.cc b/third_party/blink/renderer/core/paint/svg_shape_painter.cc
index ea10cbf775b73..e84d754be4898 100644
--- a/third_party/blink/renderer/core/paint/svg_shape_painter.cc
+++ b/third_party/blink/renderer/core/paint/svg_shape_painter.cc
@@ -4,6 +4,8 @@
 
 #include "third_party/blink/renderer/core/paint/svg_shape_painter.h"
 
+#include "base/debug/dump_without_crashing.h"
+#include "base/logging.h"
 #include "base/types/optional_util.h"
 #include "third_party/blink/renderer/core/layout/svg/layout_svg_resource_marker.h"
 #include "third_party/blink/renderer/core/layout/svg/layout_svg_shape.h"
@@ -285,6 +287,23 @@ void SVGShapePainter::PaintMarker(const PaintInfo& paint_info,
                                   float stroke_width) {
   marker.ClearInvalidationMask();
 
+  static int paint_count = 0;
+  if (++paint_count <= 5) {
+    LOG(ERROR) << "[POC] PaintMarker called (#" << paint_count
+               << "), marker NeedsLayout=" << marker.NeedsLayout()
+               << " doc=" << marker.GetDocument().Url().GetString().Utf8();
+  }
+  // [POC] Diagnostic: external resource markers are painted without re-running
+  // the resource document's lifecycle. If anything dirtied the marker subtree
+  // after SVGResourceDocumentContent::LoadingFinished(), we'd be painting a
+  // dirty NG layout subtree here.
+  if (marker.NeedsLayout()) {
+    LOG(ERROR) << "[POC][HIT] PaintMarker: marker subtree NeedsLayout()! "
+               << "marker=" << &marker << " in doc="
+               << &marker.GetDocument();
+    base::debug::DumpWithoutCrashing();
+  }
+
   if (!marker.ShouldPaint())
     return;
 
diff --git a/third_party/blink/renderer/core/svg/svg_fe_image_element.cc b/third_party/blink/renderer/core/svg/svg_fe_image_element.cc
index 3434520ff6e28..f5f212d888a87 100644
--- a/third_party/blink/renderer/core/svg/svg_fe_image_element.cc
+++ b/third_party/blink/renderer/core/svg/svg_fe_image_element.cc
@@ -21,6 +21,7 @@
 
 #include "third_party/blink/renderer/core/svg/svg_fe_image_element.h"
 
+#include "base/logging.h"
 #include "third_party/blink/renderer/core/dom/document.h"
 #include "third_party/blink/renderer/core/dom/id_target_observer.h"
 #include "third_party/blink/renderer/core/execution_context/execution_context.h"
@@ -149,6 +150,10 @@ void SVGFEImageElement::RemovedFrom(ContainerNode& root_parent) {
 }
 
 void SVGFEImageElement::ImageNotifyFinished(ImageResourceContent*) {
+  LOG(ERROR) << "[POC] SVGFEImageElement::ImageNotifyFinished doc="
+             << GetDocument().Url().GetString().Utf8()
+             << " load_event_finished="
+             << GetDocument().LoadEventFinished();
   if (!isConnected())
     return;
 
diff --git a/third_party/blink/renderer/core/svg/svg_resource_document_content.cc b/third_party/blink/renderer/core/svg/svg_resource_document_content.cc
index fef101ebe80b7..eb55386ca8abe 100644
--- a/third_party/blink/renderer/core/svg/svg_resource_document_content.cc
+++ b/third_party/blink/renderer/core/svg/svg_resource_document_content.cc
@@ -22,9 +22,11 @@
 
 #include "third_party/blink/renderer/core/svg/svg_resource_document_content.h"
 
+#include "base/logging.h"
 #include "base/notreached.h"
 #include "base/task/single_thread_task_runner.h"
 #include "third_party/blink/renderer/core/frame/local_frame.h"
+#include "third_party/blink/renderer/core/frame/local_frame_view.h"
 #include "third_party/blink/renderer/core/loader/resource/svg_document_resource.h"
 #include "third_party/blink/renderer/core/page/page.h"
 #include "third_party/blink/renderer/core/svg/graphics/isolated_svg_document_host.h"
@@ -146,14 +148,22 @@ SVGResourceDocumentContent::UpdateDocument(scoped_refptr<SharedBuffer> data,
   // so we can check if we have a usable document and notify our listeners. If
   // not, then we need to wait for the async load completion callback.
   if (!document_host_->IsLoaded()) {
+    LOG(ERROR) << "[POC] SVGResourceDocumentContent::UpdateDocument: "
+               << "going ASYNC, request_url="
+               << request_url.GetString().Utf8();
     return UpdateResult::kAsync;
   }
+  LOG(ERROR) << "[POC] SVGResourceDocumentContent::UpdateDocument: "
+             << "completed SYNC, request_url="
+             << request_url.GetString().Utf8();
   LoadingFinished();
   return UpdateResult::kCompleted;
 }
 
 void SVGResourceDocumentContent::LoadingFinished() {
   LocalFrame* frame = document_host_->GetFrame();
+  LOG(ERROR) << "[POC] SVGResourceDocumentContent::LoadingFinished url="
+             << url_.GetString().Utf8() << " (single lifecycle pass)";
   frame->View()->UpdateAllLifecyclePhasesExceptPaint(
       DocumentUpdateReason::kSVGImage);
   UpdateStatus(ResourceStatus::kCached);
@@ -241,6 +251,14 @@ SVGResourceTarget* SVGResourceDocumentContent::GetResourceTarget(
 }
 
 void SVGResourceDocumentContent::ContentChanged() {
+  if (status_ == ResourceStatus::kCached) {
+    LocalFrame* frame =
+        document_host_ ? document_host_->GetFrame() : nullptr;
+    bool dirty = frame && frame->View() && frame->View()->NeedsLayout();
+    LOG(ERROR) << "[POC] SVGResourceDocumentContent::ContentChanged "
+               << "POST-LOAD url=" << url_.GetString().Utf8()
+               << " inner_view_needs_layout=" << dirty;
+  }
   for (auto& observer : observers_) {
     observer->ResourceContentChanged(this);
   }