Backed out changeset f91e5fbb9520 (bug 1557983) as requested by dev.

layout/printing/nsPrintJob.cpp

@@ -381,6 +381,37 @@ static nsresult EnsureSettingsHasPrinterNameSet(
 #endif
 }
 
+static bool DocHasPrintCallbackCanvas(Document& aDoc) {
+  Element* root = aDoc.GetRootElement();
+  if (!root) {
+    return false;
+  }
+  // FIXME(emilio): This doesn't account for shadow dom and it's unnecessarily
+  // inefficient. Though I guess it doesn't really matter.
+  RefPtr<nsContentList> canvases =
+      NS_GetContentList(root, kNameSpaceID_XHTML, u"canvas"_ns);
+  uint32_t canvasCount = canvases->Length(true);
+  for (uint32_t i = 0; i < canvasCount; ++i) {
+    auto* canvas = HTMLCanvasElement::FromNodeOrNull(canvases->Item(i, false));
+    if (canvas && canvas->GetMozPrintCallback()) {
+      return true;
+    }
+  }
+
+  bool result = false;
+
+  auto checkSubDoc = [&result](Document& aSubDoc) {
+    if (DocHasPrintCallbackCanvas(aSubDoc)) {
+      result = true;
+      return CallState::Stop;
+    }
+    return CallState::Continue;
+  };
+
+  aDoc.EnumerateSubDocuments(checkSubDoc);
+  return result;
+}
+
 //-------------------------------------------------------
 
 NS_IMPL_ISUPPORTS(nsPrintJob, nsIWebProgressListener, nsISupportsWeakReference,
@@ -450,6 +481,8 @@ nsresult nsPrintJob::Initialize(nsIDocumentViewerPrint* aDocViewerPrint,
     }
   }
 
+  mHasMozPrintCallback = DocHasPrintCallbackCanvas(*aOriginalDoc);
+
   return NS_OK;
 }
 

layout/printing/nsPrintJob.h

@@ -131,6 +131,12 @@ class nsPrintJob final : public nsIObserver,
 
   void TurnScriptingOn(bool aDoTurnOn);
 
+  /**
+   * Checks to see if the document this print engine is associated with has any
+   * canvases that have a mozPrintCallback.
+   * https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement#Properties
+   */
+  bool HasPrintCallbackCanvas() { return mHasMozPrintCallback; }
   bool PrePrintPage();
   bool PrintPage(nsPrintObject* aPOect, bool& aInRange);
   bool DonePrintingPages(nsPrintObject* aPO, nsresult aResult);