LibWeb+WebContent: Don't relayout page on every scroll event

The WebContent process was redoing page layout every time you scrolled
the page. This was a huge CPU hog for no reason. Fix this by only doing
a relayout when the viewport is resized, not when it moves around.

3 files changed, 24 insertions, 4 deletions

diff --git a/Userland/Libraries/LibWeb/Page/Frame.cpp b/Userland/Libraries/LibWeb/Page/Frame.cpp
index 29decc7d08..ac1cbb8c4c 100644
--- a/Userland/Libraries/LibWeb/Page/Frame.cpp
+++ b/Userland/Libraries/LibWeb/Page/Frame.cpp
@@ -108,6 +108,28 @@ void Frame::set_document(DOM::Document* document)
         m_page->client().page_did_set_document_in_main_frame(m_document);
 }
 
+void Frame::set_viewport_rect(const Gfx::IntRect& rect)
+{
+    bool did_change = false;
+
+    if (m_size != rect.size()) {
+        m_size = rect.size();
+        if (m_document)
+            m_document->update_layout();
+        did_change = true;
+    }
+
+    if (m_viewport_scroll_offset != rect.location()) {
+        m_viewport_scroll_offset = rect.location();
+        did_change = true;
+    }
+
+    if (did_change) {
+        for (auto* client : m_viewport_clients)
+            client->frame_did_set_viewport_rect(rect);
+    }
+}
+
 void Frame::set_size(const Gfx::IntSize& size)
 {
     if (m_size == size)
diff --git a/Userland/Libraries/LibWeb/Page/Frame.h b/Userland/Libraries/LibWeb/Page/Frame.h
index aad2e566b8..eae2e89692 100644
--- a/Userland/Libraries/LibWeb/Page/Frame.h
+++ b/Userland/Libraries/LibWeb/Page/Frame.h
@@ -73,6 +73,7 @@ public:
 
     void set_viewport_scroll_offset(const Gfx::IntPoint&);
     Gfx::IntRect viewport_rect() const { return { m_viewport_scroll_offset, m_size }; }
+    void set_viewport_rect(const Gfx::IntRect&);
 
     void did_scroll(Badge<InProcessWebView>);
 
diff --git a/Userland/Services/WebContent/PageHost.cpp b/Userland/Services/WebContent/PageHost.cpp
index d37ff61054..929cfac044 100644
--- a/Userland/Services/WebContent/PageHost.cpp
+++ b/Userland/Services/WebContent/PageHost.cpp
@@ -100,10 +100,7 @@ void PageHost::paint(const Gfx::IntRect& content_rect, Gfx::Bitmap& target)
 
 void PageHost::set_viewport_rect(const Gfx::IntRect& rect)
 {
-    page().main_frame().set_size(rect.size());
-    if (page().main_frame().document())
-        page().main_frame().document()->update_layout();
-    page().main_frame().set_viewport_scroll_offset(rect.location());
+    page().main_frame().set_viewport_rect(rect);
 }
 
 void PageHost::page_did_invalidate(const Gfx::IntRect& content_rect)