167649 2017-01-31 09:52:08 -0800 [Cocoa] Assertion failure in contentZoomScale(WKWebView *) (scale == [webView->_scrollView zoomScale]) in iOS Mail 2017-01-31 11:21:08 -0800 1 1 1 Unclassified WebKit WebKit2 Other Unspecified Unspecified NEW InRadar P2 Normal --- 1 mitz webkit-unassigned simon.fraser thorton oldest_to_newest 1271760 0 mitz 2017-01-31 09:52:08 -0800 Running the iOS Mail app with a Debug build of WebKit, some of its web views used for message display are constantly failing the ASSERT(scale == [webView->_scrollView zoomScale]) in WKWebView.mm’s contentZoomScale(WKWebView *). In a typical example, one sees (lldb) p scale (CGFloat) $0 = 0.25 (lldb) p [(UIScrollView*)webView->_scrollView.m_ptr zoomScale] (CGFloat) $1 = 1 1271764 1 mitz 2017-01-31 09:56:33 -0800 <rdar://problem/30287825> 1271799 2 mitz 2017-01-31 11:21:08 -0800 More information from Simon: (lldb) po [self recursiveDescription] <: 0x7f91b3728f10; frame = (0 0; 363 276); gestureRecognizers = <NSArray: 0x620000052240>; layer = <CALayer: 0x6180002226c0>> | <UIView: 0x7f91b3730a80; frame = (0 0; 0 0); layer = <CALayer: 0x6180002237c0>> | <WKWebView: 0x7f91b481fc00; frame = (0 0; 363 276); autoresize = W+H; layer = <CALayer: 0x6180002219c0>> | | <WKScrollView: 0x7f91b683e800; baseClass = UIScrollView; frame = (0 0; 363 276); clipsToBounds = YES; gestureRecognizers = <NSArray: 0x600000056650>; layer = <CALayer: 0x600000221420>; contentOffset: {0, 0}; contentSize: {0, 0}> | | | <WKContentView: 0x7f91b6813800; frame = (0 0; 0 0); transform = [0.25, 0, 0, 0.25, 0, 0]; gestureRecognizers = <NSArray: 0x64000004b040>; layer = <CALayer: 0x6000002213e0>> | | | | <UIView: 0x7f91b5c0f8f0; frame = (0 0; 0 0); clipsToBounds = YES; layer = <CALayer: 0x64000003b060>> | | | | | <UIView: 0x7f91b5c0f750; frame = (0 0; 0 0); autoresize = W+H; layer = <CALayer: 0x64000003aea0>> | | | | | | <WKCompositingView: 0x7f91b5b250e0; frame = (0 0; 0 0); layer = <CALayer: 0x62800023e540>> layerID = 1 "<WKCompositingView: 0x7f91b5b250e0>" | | | <UIView: 0x7f91b5c0fa90; frame = (0 0; 0 0); opaque = NO; layer = <CALayer: 0x64000003b020>> | | | | <UITextSelectionView: 0x7f91b5b253f0; frame = (0 0; 0 0); userInteractionEnabled = NO; layer = <CALayer: 0x62800023e560>> | | | <: 0x7f91b37428a0; frame = (0 0; 0 0); userInteractionEnabled = NO; layer = <CALayer: 0x618000222b00>> | <: 0x7f91b3741a10; baseClass = UIStackView; frame = (0 0; 0 0); layer = <CATransformLayer: 0x618000222a40>> The layer transform is being set via -[UIScrollView setZoomScale:] being called from: frame #13: WebKit`::-[WKWebView _didCommitLayerTree:](self=0x00007fb8b00aea00, _cmd="_didCommitLayerTree:", layerTreeTransaction=0x00007fff55f11a00) at WKWebView.mm:1271 frame #14: WebKit`::-[WKContentView _didCommitLayerTree:](self=0x00007fb8b00b5400, _cmd="_didCommitLayerTree:", layerTreeTransaction=0x00007fff55f11a00) at WKContentView.mm:562 frame #15: WebKit`WebKit::PageClientImpl::didCommitLayerTree(this=0x0000600000422dc0, layerTreeTransaction=0x00007fff55f11a00) at PageClientImplIOS.mm:506 frame #16: WebKit`WebKit::WebPageProxy::didCommitLayerTree(this=0x00007fb8b000c018, layerTreeTransaction=0x00007fff55f11a00) at WebPageProxyIOS.mm:407 frame #17: WebKit`WebKit::RemoteLayerTreeDrawingAreaProxy::commitLayerTree(this=0x00007fb8b41020d0, layerTreeTransaction=0x00007fff55f11a00, scrollingTreeTransaction=0x00007fff55f11b08) at RemoteLayerTreeDrawingAreaProxy.mm:200 but this seems to happen for an offscreen WKWebView (I see this before viewing any message on iPhone). Then the assertion hits when the WKWebView is being resized. My guess is that the 0.25x scale is some WKWebView content autosizing minimum scale, and the assertion hits when the WKWebView is being prepared for display.