From 15e35b0d716ea8fe2fab7a9607902dedc978688a Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Wed, 2 Dec 2020 23:40:57 +0100
Subject: [PATCH] LibWeb: Layout viewport rect was lagging behind when resizing

Layout was using an outdated viewport rect that we set *after* doing
a layout due to resize. That meant that layout-in-response-to-resize
was always lagging behind the current size of the view.

The root of this problem was how Frame kept both a viewport rect
(with both scroll offset and size) and a frame size. To fix this,
only store the viewport scroll offset, and always use the frame size.
This way they can't get out of sync and the problem goes away. :^)

Fixes #4250.
---
 Libraries/LibWeb/InProcessWebView.cpp |  4 ++--
 Libraries/LibWeb/Page/Frame.cpp       | 10 +++++-----
 Libraries/LibWeb/Page/Frame.h         |  6 +++---
 Services/WebContent/PageHost.cpp      |  2 +-
 4 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/Libraries/LibWeb/InProcessWebView.cpp b/Libraries/LibWeb/InProcessWebView.cpp
index 88fe7670090..c878f20750f 100644
--- a/Libraries/LibWeb/InProcessWebView.cpp
+++ b/Libraries/LibWeb/InProcessWebView.cpp
@@ -248,7 +248,7 @@ void InProcessWebView::layout_and_sync_size()
         set_content_size(layout_root()->size().to_type<int>());
     }
 
-    page().main_frame().set_viewport_rect(viewport_rect_in_content_coordinates());
+    page().main_frame().set_viewport_scroll_offset({ horizontal_scrollbar().value(), vertical_scrollbar().value() });
 }
 
 void InProcessWebView::resize_event(GUI::ResizeEvent& event)
@@ -409,7 +409,7 @@ void InProcessWebView::set_document(DOM::Document* document)
 
 void InProcessWebView::did_scroll()
 {
-    page().main_frame().set_viewport_rect(viewport_rect_in_content_coordinates());
+    page().main_frame().set_viewport_scroll_offset({ horizontal_scrollbar().value(), vertical_scrollbar().value() });
     page().main_frame().did_scroll({});
 }
 
diff --git a/Libraries/LibWeb/Page/Frame.cpp b/Libraries/LibWeb/Page/Frame.cpp
index c9e0a3e6cdc..333425f8acf 100644
--- a/Libraries/LibWeb/Page/Frame.cpp
+++ b/Libraries/LibWeb/Page/Frame.cpp
@@ -106,19 +106,19 @@ void Frame::set_size(const Gfx::IntSize& size)
         m_document->layout();
 }
 
-void Frame::set_viewport_rect(const Gfx::IntRect& rect)
+void Frame::set_viewport_scroll_offset(const Gfx::IntPoint& offset)
 {
-    if (m_viewport_rect == rect)
+    if (m_viewport_scroll_offset == offset)
         return;
-    m_viewport_rect = rect;
+    m_viewport_scroll_offset = offset;
 
     if (m_document && m_document->layout_node())
-        m_document->layout_node()->did_set_viewport_rect({}, rect);
+        m_document->layout_node()->did_set_viewport_rect({}, viewport_rect());
 }
 
 void Frame::set_needs_display(const Gfx::IntRect& rect)
 {
-    if (!m_viewport_rect.intersects(rect))
+    if (!viewport_rect().intersects(rect))
         return;
 
     if (is_main_frame()) {
diff --git a/Libraries/LibWeb/Page/Frame.h b/Libraries/LibWeb/Page/Frame.h
index 2696454182b..6e0a336bf17 100644
--- a/Libraries/LibWeb/Page/Frame.h
+++ b/Libraries/LibWeb/Page/Frame.h
@@ -63,8 +63,8 @@ public:
 
     void set_needs_display(const Gfx::IntRect&);
 
-    void set_viewport_rect(const Gfx::IntRect&);
-    Gfx::IntRect viewport_rect() const { return m_viewport_rect; }
+    void set_viewport_scroll_offset(const Gfx::IntPoint&);
+    Gfx::IntRect viewport_rect() const { return { m_viewport_scroll_offset, m_size }; }
 
     void did_scroll(Badge<InProcessWebView>);
 
@@ -109,7 +109,7 @@ private:
     WeakPtr<DOM::Element> m_host_element;
     RefPtr<DOM::Document> m_document;
     Gfx::IntSize m_size;
-    Gfx::IntRect m_viewport_rect;
+    Gfx::IntPoint m_viewport_scroll_offset;
 
     DOM::Position m_cursor_position;
     RefPtr<Core::Timer> m_cursor_blink_timer;
diff --git a/Services/WebContent/PageHost.cpp b/Services/WebContent/PageHost.cpp
index a23cf0c764d..5e8952143c9 100644
--- a/Services/WebContent/PageHost.cpp
+++ b/Services/WebContent/PageHost.cpp
@@ -103,7 +103,7 @@ 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()->layout();
-    page().main_frame().set_viewport_rect(rect);
+    page().main_frame().set_viewport_scroll_offset(rect.location());
 }
 
 void PageHost::page_did_invalidate(const Gfx::IntRect& content_rect)