Document UnifiedPDFPlugin coordinate spaces

https://bugs.webkit.org/show_bug.cgi?id=265803

Reviewed by Wenson Hsieh and Richard Robinson.

* Source/WebKit/WebProcess/Plugins/PDF/UnifiedPDF/UnifiedPDFPlugin.h:

Canonical link: https://commits.webkit.org/271506@main

Source/WebKit/WebProcess/Plugins/PDF/UnifiedPDF/UnifiedPDFPlugin.h

@@ -72,6 +72,24 @@ class UnifiedPDFPlugin final : public PDFPluginBase, public WebCore::GraphicsLay
     void didEndMagnificationGesture() override;
     void setPageScaleFactor(double scale, std::optional<WebCore::IntPoint> origin) final;
 
+    /*
+        Unified PDF Plugin coordinate spaces, in depth order:
+
+        - "root view": same as the rest of WebKit.
+
+        - "plugin": the space of the plugin element (origin at the top left,
+            ignoring all internal transforms).
+
+        - "contents": the space of the contents layer, with scrolling subtracted
+            out and page scale multiplied in; the painting space.
+
+        - "document": the space that the PDF pages are laid down in, with
+            PDFDocumentLayout's width-fitting scale divided out; includes margins.
+
+        - "page": the space of each actual PDFPage, as used by PDFKit; origin at
+            the bottom left of the crop box; page rotation multiplied in.
+    */
+
     WebCore::IntSize documentSize() const;
     WebCore::IntSize contentsSize() const override;
     unsigned firstPageHeight() const override;