diff --git a/LayoutTests/ChangeLog b/LayoutTests/ChangeLog
index f29d923..836374a 100644
--- a/LayoutTests/ChangeLog
+++ b/LayoutTests/ChangeLog
@@ -1,5 +1,24 @@
 2019-05-11  Simon Fraser  <simon.fraser@apple.com>
 
+        Layer bounds are incorrect for sharing layers that paint with transforms
+        https://bugs.webkit.org/show_bug.cgi?id=197768
+        <rdar://problem/50695493>
+
+        Reviewed by Zalan Bujtas.
+
+        Tests for backing-shared layer bounds in various configurations.
+
+        * compositing/shared-backing/sharing-bounds-clip-expected.txt: Added.
+        * compositing/shared-backing/sharing-bounds-clip.html: Added.
+        * compositing/shared-backing/sharing-bounds-expected.txt: Added.
+        * compositing/shared-backing/sharing-bounds-non-clipping-shared-layer-expected.txt: Added.
+        * compositing/shared-backing/sharing-bounds-non-clipping-shared-layer.html: Added.
+        * compositing/shared-backing/sharing-bounds-transformed-sharing-layer-expected.txt: Added.
+        * compositing/shared-backing/sharing-bounds-transformed-sharing-layer.html: Added.
+        * compositing/shared-backing/sharing-bounds.html: Added.
+
+2019-05-11  Simon Fraser  <simon.fraser@apple.com>
+
         Translucent gradient rendering bug due to will-change transform
         https://bugs.webkit.org/show_bug.cgi?id=197654
         <rdar://problem/50547664>
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds-clip-expected.txt b/LayoutTests/compositing/shared-backing/sharing-bounds-clip-expected.txt
new file mode 100644
index 0000000..67d5ffc
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds-clip-expected.txt
@@ -0,0 +1,23 @@
+(GraphicsLayer
+  (anchor 0.00 0.00)
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (children 2
+        (GraphicsLayer
+          (position 8.00 8.00)
+          (bounds 50.00 50.00)
+          (contentsOpaque 1)
+        )
+        (GraphicsLayer
+          (position 30.00 30.00)
+          (bounds 500.00 302.00)
+          (drawsContent 1)
+        )
+      )
+    )
+  )
+)
+
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds-clip.html b/LayoutTests/compositing/shared-backing/sharing-bounds-clip.html
new file mode 100644
index 0000000..06a37b9
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds-clip.html
@@ -0,0 +1,63 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>Tests the backing provider layer bounds with a transformed sharing layer</title>
+    <style>
+        .relative {
+            position: absolute;
+            top: 20px;
+            left: 20px;
+            height: 300px;
+            width: 300px;
+            margin: 10px;
+            border: 1px solid black;
+            background-color: silver;
+            clip: rect(10px, 500px, 300px, 10px); /* larger than the bounds */
+        }
+        
+        .sharing {
+			position: relative;
+			top: 50px;
+			left: 50px;
+            width: 200px;
+            height: 200px;
+            background-color: green;
+        }
+        
+        .inner {
+            position: absolute;
+            left: 200px;
+            top: 200px;
+            width: 100px;
+            height: 100px;
+            background-color: blue;
+        }
+
+        .trigger {
+            transform: translateZ(0);
+            width: 50px;
+            height: 50px;
+            background-color: silver;
+        }
+    </style>
+    <script>
+        if (window.testRunner)
+            testRunner.dumpAsText();
+
+        window.addEventListener('load', () => {
+            if (window.internals)
+                document.getElementById('layers').textContent = internals.layerTreeAsText(document);
+        }, false);
+    </script>
+</head>
+<body>
+    <div class="trigger"></div>
+    <div class="relative">
+        <div class="sharing">
+            <div class="inner">
+            </div>
+        </div>
+    </div>
+<pre id="layers"></pre>
+</body>
+</html>
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds-expected.txt b/LayoutTests/compositing/shared-backing/sharing-bounds-expected.txt
new file mode 100644
index 0000000..3508f59
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds-expected.txt
@@ -0,0 +1,23 @@
+(GraphicsLayer
+  (anchor 0.00 0.00)
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (children 2
+        (GraphicsLayer
+          (position 8.00 8.00)
+          (bounds 50.00 50.00)
+          (contentsOpaque 1)
+        )
+        (GraphicsLayer
+          (position 30.00 30.00)
+          (bounds 302.00 302.00)
+          (drawsContent 1)
+        )
+      )
+    )
+  )
+)
+
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds-non-clipping-shared-layer-expected.txt b/LayoutTests/compositing/shared-backing/sharing-bounds-non-clipping-shared-layer-expected.txt
new file mode 100644
index 0000000..f9b9b6a
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds-non-clipping-shared-layer-expected.txt
@@ -0,0 +1,23 @@
+(GraphicsLayer
+  (anchor 0.00 0.00)
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (children 2
+        (GraphicsLayer
+          (position 8.00 8.00)
+          (bounds 50.00 50.00)
+          (contentsOpaque 1)
+        )
+        (GraphicsLayer
+          (position 30.00 30.00)
+          (bounds 501.00 501.00)
+          (drawsContent 1)
+        )
+      )
+    )
+  )
+)
+
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds-non-clipping-shared-layer.html b/LayoutTests/compositing/shared-backing/sharing-bounds-non-clipping-shared-layer.html
new file mode 100644
index 0000000..3c26c86
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds-non-clipping-shared-layer.html
@@ -0,0 +1,63 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>Tests the backing provider layer bounds with a transformed sharing layer</title>
+    <style>
+        .relative {
+            position: absolute;
+            top: 20px;
+            left: 20px;
+            height: 300px;
+            width: 300px;
+            margin: 10px;
+            border: 1px solid black;
+        }
+        
+        .sharing {
+			position: relative;
+			top: 50px;
+			left: 50px;
+            width: 200px;
+            height: 200px;
+            background-color: green;
+            transform: scale(1.5);
+            transform-origin: top left;
+        }
+        
+        .inner {
+            position: absolute;
+            left: 200px;
+            top: 200px;
+            width: 100px;
+            height: 100px;
+            background-color: blue;
+        }
+
+        .trigger {
+            transform: translateZ(0);
+            width: 50px;
+            height: 50px;
+            background-color: silver;
+        }
+    </style>
+    <script>
+        if (window.testRunner)
+            testRunner.dumpAsText();
+
+        window.addEventListener('load', () => {
+            if (window.internals)
+                document.getElementById('layers').textContent = internals.layerTreeAsText(document);
+        }, false);
+    </script>
+</head>
+<body>
+    <div class="trigger"></div>
+    <div class="relative">
+        <div class="sharing">
+            <div class="inner">
+            </div>
+        </div>
+    </div>
+<pre id="layers"></pre>
+</body>
+</html>
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds-transformed-sharing-layer-expected.txt b/LayoutTests/compositing/shared-backing/sharing-bounds-transformed-sharing-layer-expected.txt
new file mode 100644
index 0000000..3508f59
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds-transformed-sharing-layer-expected.txt
@@ -0,0 +1,23 @@
+(GraphicsLayer
+  (anchor 0.00 0.00)
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (children 2
+        (GraphicsLayer
+          (position 8.00 8.00)
+          (bounds 50.00 50.00)
+          (contentsOpaque 1)
+        )
+        (GraphicsLayer
+          (position 30.00 30.00)
+          (bounds 302.00 302.00)
+          (drawsContent 1)
+        )
+      )
+    )
+  )
+)
+
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds-transformed-sharing-layer.html b/LayoutTests/compositing/shared-backing/sharing-bounds-transformed-sharing-layer.html
new file mode 100644
index 0000000..0d08d15
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds-transformed-sharing-layer.html
@@ -0,0 +1,50 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>Tests bounds computation for a transformed layer painting into shared backing</title>
+    <style>
+        .clipping {
+            position: absolute;
+            top: 20px;
+            left: 20px;
+            overflow: hidden;
+            height: 300px;
+            width: 300px;
+            margin: 10px;
+            border: 1px solid black;
+        }
+
+        .transformed {
+            width: 400px;
+            height: 100px;
+            background-color: green;
+            transform: scale(2);
+            transform-origin: top left;
+        }
+
+        .trigger {
+            transform: translateZ(0);
+            width: 50px;
+            height: 50px;
+            background-color: silver;
+        }
+    </style>
+    <script>
+        if (window.testRunner)
+            testRunner.dumpAsText();
+
+        window.addEventListener('load', () => {
+            if (window.internals)
+                document.getElementById('layers').textContent = internals.layerTreeAsText(document);
+        }, false);
+    </script>
+</head>
+<body>
+    <div class="trigger"></div>
+    <div class="clipping">
+        <div class="transformed">
+        </div>
+    </div>
+<pre id="layers"></pre>
+</body>
+</html>
diff --git a/LayoutTests/compositing/shared-backing/sharing-bounds.html b/LayoutTests/compositing/shared-backing/sharing-bounds.html
new file mode 100644
index 0000000..33a6240
--- /dev/null
+++ b/LayoutTests/compositing/shared-backing/sharing-bounds.html
@@ -0,0 +1,63 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>Tests the backing provider layer bounds with a positioned child</title>
+    <style>
+        .clipping {
+            position: absolute;
+            top: 20px;
+            left: 20px;
+            overflow: hidden;
+            height: 300px;
+            width: 300px;
+            margin: 10px;
+            border: 1px solid black;
+        }
+        
+        .sharing {
+			position: relative;
+			top: 50px;
+			left: 50px;
+            width: 200px;
+            height: 200px;
+            background-color: green;
+        }
+        
+        .inner {
+            position: absolute;
+            left: 200px;
+            top: 200px;
+            width: 100px;
+            height: 100px;
+            background-color: blue;
+        }
+
+        .trigger {
+            transform: translateZ(0);
+            width: 50px;
+            height: 50px;
+            background-color: silver;
+        }
+        
+    </style>
+    <script>
+        if (window.testRunner)
+            testRunner.dumpAsText();
+
+        window.addEventListener('load', () => {
+            if (window.internals)
+                document.getElementById('layers').textContent = internals.layerTreeAsText(document);
+        }, false);
+    </script>
+</head>
+<body>
+    <div class="composited trigger"></div>
+    <div class="clipping">
+        <div class="sharing">
+            <div class="inner">
+            </div>
+        </div>
+    </div>
+<pre id="layers"></pre>
+</body>
+</html>
diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index b5e803e..c7c8dca 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,5 +1,27 @@
 2019-05-11  Simon Fraser  <simon.fraser@apple.com>
 
+        Layer bounds are incorrect for sharing layers that paint with transforms
+        https://bugs.webkit.org/show_bug.cgi?id=197768
+        <rdar://problem/50695493>
+
+        Reviewed by Zalan Bujtas.
+
+        We don't need to traverse shared layers if the backing-provider has overflow clip,
+        because we know they are containing-block descendants and therefore clipped.
+
+        Note tha the CSS "clip" property doesn't guarantee this, because the clip rect
+        can be larger than the element, so in that case we just traverse shared layers.
+
+        Tests: compositing/shared-backing/sharing-bounds-clip.html
+               compositing/shared-backing/sharing-bounds-non-clipping-shared-layer.html
+               compositing/shared-backing/sharing-bounds-transformed-sharing-layer.html
+               compositing/shared-backing/sharing-bounds.html
+
+        * rendering/RenderLayerBacking.cpp:
+        (WebCore::RenderLayerBacking::updateCompositedBounds):
+
+2019-05-11  Simon Fraser  <simon.fraser@apple.com>
+
         Translucent gradient rendering bug due to will-change transform
         https://bugs.webkit.org/show_bug.cgi?id=197654
         <rdar://problem/50547664>
diff --git a/Source/WebCore/rendering/RenderLayerBacking.cpp b/Source/WebCore/rendering/RenderLayerBacking.cpp
index 205b77b..43d1a57 100644
--- a/Source/WebCore/rendering/RenderLayerBacking.cpp
+++ b/Source/WebCore/rendering/RenderLayerBacking.cpp
@@ -671,12 +671,15 @@
         layerBounds.intersect(clippingBounds);
     }
 
-    for (auto& layerWeakPtr : m_backingSharingLayers) {
-        auto* boundsRootLayer = &m_owningLayer;
-        ASSERT(layerWeakPtr->isDescendantOf(m_owningLayer));
-        auto offset = layerWeakPtr->offsetFromAncestor(&m_owningLayer);
-        auto bounds = layerWeakPtr->calculateLayerBounds(boundsRootLayer, offset, RenderLayer::defaultCalculateLayerBoundsFlags() | RenderLayer::ExcludeHiddenDescendants | RenderLayer::DontConstrainForMask);
-        layerBounds.unite(bounds);
+    // If the backing provider has overflow:clip, we know all sharing layers are affected by the clip because they are containing-block descendants.
+    if (!renderer().hasOverflowClip()) {
+        for (auto& layerWeakPtr : m_backingSharingLayers) {
+            auto* boundsRootLayer = &m_owningLayer;
+            ASSERT(layerWeakPtr->isDescendantOf(m_owningLayer));
+            auto offset = layerWeakPtr->offsetFromAncestor(&m_owningLayer);
+            auto bounds = layerWeakPtr->calculateLayerBounds(boundsRootLayer, offset, RenderLayer::defaultCalculateLayerBoundsFlags() | RenderLayer::ExcludeHiddenDescendants | RenderLayer::DontConstrainForMask);
+            layerBounds.unite(bounds);
+        }
     }
 
     // If the element has a transform-origin that has fixed lengths, and the renderer has zero size,