LibHTML: Mark image bitmaps outside the visible viewport as volatile

When the visible viewport rect changes, we walk the layout tree and
check where each LayoutImage is in relation to the viewport rect.
Images outside have their bitmaps marked as volatile.

Note that the bitmaps are managed by ImageDecoder objects. If a bitmap
is purged by the kernel while volatile, we construct a new ImageDecoder
next time we need pixels for the image.

Libraries/LibHTML/DOM/HTMLImageElement.cpp

@@ -80,3 +80,17 @@ const GraphicsBitmap* HTMLImageElement::bitmap() const
         return nullptr;
     return m_image_decoder->bitmap();
 }
+
+void HTMLImageElement::set_volatile(Badge<LayoutDocument>, bool v)
+{
+    if (!m_image_decoder)
+        return;
+    if (v) {
+        m_image_decoder->set_volatile();
+        return;
+    }
+    bool has_image = m_image_decoder->set_nonvolatile();
+    if (has_image)
+        return;
+    m_image_decoder = ImageDecoder::create(m_encoded_data.data(), m_encoded_data.size());
+}

Libraries/LibHTML/DOM/HTMLImageElement.h

@@ -4,6 +4,8 @@
 #include <LibDraw/ImageDecoder.h>
 #include <LibHTML/DOM/HTMLElement.h>
 
+class LayoutDocument;
+
 class HTMLImageElement : public HTMLElement {
 public:
     HTMLImageElement(Document&, const String& tag_name);
@@ -19,12 +21,13 @@ public:
     const GraphicsBitmap* bitmap() const;
     const ImageDecoder* image_decoder() const { return m_image_decoder; }
 
+    void set_volatile(Badge<LayoutDocument>, bool);
+
 private:
     void load_image(const String& src);
 
     virtual RefPtr<LayoutNode> create_layout_node(const StyleProperties* parent_style) const override;
 
     RefPtr<ImageDecoder> m_image_decoder;
-    mutable RefPtr<GraphicsBitmap> m_bitmap;
     ByteBuffer m_encoded_data;
 };

Libraries/LibHTML/Frame.cpp

@@ -1,6 +1,7 @@
 #include <LibHTML/DOM/Document.h>
 #include <LibHTML/Frame.h>
 #include <LibHTML/HtmlView.h>
+#include <LibHTML/Layout/LayoutDocument.h>
 
 Frame::Frame(HtmlView& html_view)
     : m_html_view(html_view.make_weak_ptr())
@@ -37,6 +38,9 @@ void Frame::set_viewport_rect(const Rect& rect)
     if (m_viewport_rect == rect)
         return;
     m_viewport_rect = rect;
+
+    if (m_document && m_document->layout_node())
+        m_document->layout_node()->did_set_viewport_rect({}, rect);
 }
 
 void Frame::set_needs_display(const Rect& rect)

Libraries/LibHTML/Layout/LayoutDocument.cpp

@@ -1,6 +1,7 @@
-#include <LibHTML/Frame.h>
 #include <LibHTML/Dump.h>
+#include <LibHTML/Frame.h>
 #include <LibHTML/Layout/LayoutDocument.h>
+#include <LibHTML/Layout/LayoutImage.h>
 
 LayoutDocument::LayoutDocument(const Document& document, NonnullRefPtr<StyleProperties> style)
     : LayoutBlock(&document, move(style))
@@ -29,3 +30,15 @@ void LayoutDocument::layout()
     });
     rect().set_bottom(lowest_bottom);
 }
+
+void LayoutDocument::did_set_viewport_rect(Badge<Frame>, const Rect& a_viewport_rect)
+{
+    FloatRect viewport_rect(a_viewport_rect.x(), a_viewport_rect.y(), a_viewport_rect.width(), a_viewport_rect.height());
+    for_each_in_subtree([&](auto& layout_node) {
+        if (is<LayoutImage>(layout_node)) {
+            auto& image = to<LayoutImage>(layout_node);
+            const_cast<HTMLImageElement&>(image.node()).set_volatile({}, !viewport_rect.intersects(image.rect()));
+        }
+        return IterationDecision::Continue;
+    });
+}

Libraries/LibHTML/Layout/LayoutDocument.h

@@ -15,6 +15,8 @@ public:
     const LayoutRange& selection() const { return m_selection; }
     LayoutRange& selection() { return m_selection; }
 
+    void did_set_viewport_rect(Badge<Frame>, const Rect&);
+
 private:
     LayoutRange m_selection;
 };