diff --git a/LayoutTests/ChangeLog b/LayoutTests/ChangeLog
index c20588b..f29d923 100644
--- a/LayoutTests/ChangeLog
+++ b/LayoutTests/ChangeLog
@@ -1,5 +1,16 @@
 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>
+
+        Reviewed by Dean Jackson.
+
+        * compositing/contents-opaque/background-change-to-transparent-expected.txt: Added.
+        * compositing/contents-opaque/background-change-to-transparent.html: Added.
+
+2019-05-11  Simon Fraser  <simon.fraser@apple.com>
+
         When the scroller hosting a shared layer becomes non-scrollable, content disappears
         https://bugs.webkit.org/show_bug.cgi?id=197766
         <rdar://problem/50695808>
diff --git a/LayoutTests/compositing/contents-opaque/background-change-to-transparent-expected.txt b/LayoutTests/compositing/contents-opaque/background-change-to-transparent-expected.txt
new file mode 100644
index 0000000..9dc4c09
--- /dev/null
+++ b/LayoutTests/compositing/contents-opaque/background-change-to-transparent-expected.txt
@@ -0,0 +1,19 @@
+Text here
+(GraphicsLayer
+  (anchor 0.00 0.00)
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (children 1
+        (GraphicsLayer
+          (position 18.00 10.00)
+          (bounds 300.00 300.00)
+          (drawsContent 1)
+        )
+      )
+    )
+  )
+)
+
diff --git a/LayoutTests/compositing/contents-opaque/background-change-to-transparent.html b/LayoutTests/compositing/contents-opaque/background-change-to-transparent.html
new file mode 100644
index 0000000..9b2ed20
--- /dev/null
+++ b/LayoutTests/compositing/contents-opaque/background-change-to-transparent.html
@@ -0,0 +1,41 @@
+<!DOCTYPE html>
+<html>
+<head>
+    <title>Tests that contentsOpaque is re-evaluated after a background style change</title>
+    <style>
+        .composited {
+            margin: 20px;
+            height: 300px;
+            width: 300px;
+            margin: 10px;
+            background-color: silver;
+            transform: translateZ(0);
+        }
+        
+        .composited.changed {
+            background: none;
+        }
+    </style>
+    <script>
+        if (window.testRunner) {
+            testRunner.dumpAsText();
+            testRunner.waitUntilDone();
+        }
+
+        window.addEventListener('load', () => {
+            setTimeout(() => {
+                document.querySelector('.composited').classList.add('changed');
+                if (window.internals)
+                    document.getElementById('layers').textContent = window.internals.layerTreeAsText(document);
+                if (window.testRunner)
+                    testRunner.notifyDone();
+            }, 0);
+        }, false);
+    </script>
+</head>
+<body>
+    <div class="composited">Text here</div>
+<pre id="layers"></pre>
+</body>
+</html>
+
diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index e50595f..b5e803e 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,5 +1,31 @@
 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>
+
+        Reviewed by Dean Jackson.
+        
+        We failed to re-evaluate 'contentsOpaque' when a background changed, because this
+        happened in updateGeometry() and that doesn't run for background changes.
+        
+        However, 'contentsOpaque' also requires knowing about geometry because we have to
+        turn it off when there's subpixel positioning, and updateConfiguration()
+        runs before updateGeometry().
+        
+        So compute m_hasSubpixelRounding in updateGeometry() and set contentsOpaque in
+        updateAfterDescendants().
+
+        Test: compositing/contents-opaque/background-change-to-transparent.html
+
+        * rendering/RenderLayerBacking.cpp:
+        (WebCore::RenderLayerBacking::updateConfiguration):
+        (WebCore::RenderLayerBacking::updateGeometry):
+        (WebCore::RenderLayerBacking::updateAfterDescendants):
+        * rendering/RenderLayerBacking.h:
+
+2019-05-11  Simon Fraser  <simon.fraser@apple.com>
+
         When the scroller hosting a shared layer becomes non-scrollable, content disappears
         https://bugs.webkit.org/show_bug.cgi?id=197766
         <rdar://problem/50695808>
diff --git a/Source/WebCore/rendering/RenderLayerBacking.cpp b/Source/WebCore/rendering/RenderLayerBacking.cpp
index c3fcb9f..205b77b 100644
--- a/Source/WebCore/rendering/RenderLayerBacking.cpp
+++ b/Source/WebCore/rendering/RenderLayerBacking.cpp
@@ -819,7 +819,7 @@
         updateRootLayerConfiguration();
 
     updateEventRegion();
-    
+
     // Requires layout.
     if (contentsInfo.isDirectlyCompositedImage())
         updateImageContents(contentsInfo);
@@ -1112,20 +1112,13 @@
     LayoutSize oldSubpixelOffsetFromRenderer = m_subpixelOffsetFromRenderer;
     primaryGraphicsLayerOffsetFromRenderer = computeOffsetFromRenderer(-rendererOffset.fromPrimaryGraphicsLayer(), deviceScaleFactor());
     m_subpixelOffsetFromRenderer = primaryGraphicsLayerOffsetFromRenderer.m_subpixelOffset;
+    m_hasSubpixelRounding = !m_subpixelOffsetFromRenderer.isZero() || compositedBounds().size() != primaryGraphicsLayerRect.size();
 
     if (primaryGraphicsLayerOffsetFromRenderer.m_devicePixelOffset != m_graphicsLayer->offsetFromRenderer()) {
         m_graphicsLayer->setOffsetFromRenderer(primaryGraphicsLayerOffsetFromRenderer.m_devicePixelOffset);
         positionOverflowControlsLayers();
     }
 
-    if (!m_isMainFrameRenderViewLayer && !m_isFrameLayerWithTiledBacking && !m_requiresBackgroundLayer) {
-        // For non-root layers, background is always painted by the primary graphics layer.
-        ASSERT(!m_backgroundLayer);
-        // Subpixel offset from graphics layer or size changed.
-        bool hadSubpixelRounding = !m_subpixelOffsetFromRenderer.isZero() || compositedBounds().size() != primaryGraphicsLayerRect.size();
-        m_graphicsLayer->setContentsOpaque(!hadSubpixelRounding && m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
-    }
-
     // If we have a layer that clips children, position it.
     LayoutRect clippingBox;
     if (auto* clipLayer = clippingLayer()) {
@@ -1326,6 +1319,12 @@
 
     updateDrawsContent(contentsInfo);
 
+    if (!m_isMainFrameRenderViewLayer && !m_isFrameLayerWithTiledBacking && !m_requiresBackgroundLayer) {
+        // For non-root layers, background is always painted by the primary graphics layer.
+        ASSERT(!m_backgroundLayer);
+        m_graphicsLayer->setContentsOpaque(!m_hasSubpixelRounding && m_owningLayer.backgroundIsKnownToBeOpaqueInRect(compositedBounds()));
+    }
+
     m_graphicsLayer->setContentsVisible(m_owningLayer.hasVisibleContent() || hasVisibleNonCompositedDescendants());
     if (m_scrollContainerLayer) {
         m_scrollContainerLayer->setContentsVisible(renderer().style().visibility() == Visibility::Visible);
diff --git a/Source/WebCore/rendering/RenderLayerBacking.h b/Source/WebCore/rendering/RenderLayerBacking.h
index 28d5a78..132e5a3 100644
--- a/Source/WebCore/rendering/RenderLayerBacking.h
+++ b/Source/WebCore/rendering/RenderLayerBacking.h
@@ -434,6 +434,7 @@
 #endif
     bool m_backgroundLayerPaintsFixedRootBackground { false };
     bool m_requiresBackgroundLayer { false };
+    bool m_hasSubpixelRounding { false };
     bool m_paintsSubpixelAntialiasedText { false }; // This is for logging only.
 };