It seems this test has been crashing in Debug since long time ago: https://results.webkit.org/?suite=layout-tests&test=imported%2Fw3c%2Fweb-platform-tests%2Fhtml%2Fcanvas%2Foffscreen%2Ftext%2F2d.text.drawing.style.measure.rtl.text.worker.html&platform=WPE Crash: STDERR: ASSERTION FAILED: isMainThread() STDERR: /app/webkit/Source/WebCore/platform/graphics/Font.cpp(676) : bool WebCore::Font::canRenderCombiningCharacterSequence(const UChar*, size_t) const STDERR: 1 0x7f99ac067861 WTFCrash Thread 1 (Thread 0x7f988b1ff640 (LWP 51810)): #0 0x00007f99ac067866 in WTFCrash () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #1 0x00007f99a8d85a14 in std::numeric_limits<unsigned int>::max() () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #2 0x00007f99af18756a in WebCore::Font::canRenderCombiningCharacterSequence(char16_t const*, unsigned long) const () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #3 0x00007f99b00c0d34 in WebCore::FontCascade::fontForCombiningCharacterSequence(char16_t const*, unsigned long) const () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #4 0x00007f99af1183a1 in WebCore::ComplexTextController::collectComplexTextRuns() () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #5 0x00007f99af116de3 in WebCore::ComplexTextController::ComplexTextController(WebCore::FontCascade const&, WebCore::TextRun const&, bool, WTF::HashSet<WebCore::Font const*, WTF::DefaultHash<WebCore::Font const*>, WTF::HashTraits<WebCore::Font const*>, WTF::HashTableTraits>*, bool) () at /app/webkit/WebKitBuild/Debug/lib/libWPEW #6 0x00007f99af1b6451 in WebCore::FontCascade::floatWidthForComplexText(WebCore::TextRun const&, WTF::HashSet<WebCore::Font const*, WTF::DefaultHash<WebCore::Font const*>, WTF::HashTraits<WebCore::Font const*>, WTF::HashTableTraits>*, WebCore::GlyphOverflow*) const () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #7 0x00007f99af1b2717 in WebCore::FontCascade::width(WebCore::TextRun const&, WTF::HashSet<WebCore::Font const*, WTF::DefaultHash<WebCore::Font const*>, WTF::HashTraits<WebCore::Font const*>, WTF::HashTableTraits>*, WebCore::GlyphOverflow*) const () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #8 0x00007f99ae8325fa in WebCore::CanvasRenderingContext2DBase::FontProxy::width(WebCore::TextRun const&, WebCore::GlyphOverflow*) const () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #9 0x00007f99ae83dc38 in WebCore::CanvasRenderingContext2DBase::measureTextInternal(WebCore::TextRun const&) () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #10 0x00007f99ae83db80 in WebCore::CanvasRenderingContext2DBase::measureTextInternal(WTF::String const&) () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #11 0x00007f99ae862552 in WebCore::OffscreenCanvasRenderingContext2D::measureText(WTF::String const&) () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #12 0x00007f99acccf1dd in WebCore::jsOffscreenCanvasRenderingContext2DPrototypeFunction_measureTextBody(JSC::JSGlobalObject*, JSC::CallFrame*, WebCore::JSOffscreenCanvasRenderingContext2D*) () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #13 0x00007f99acce1cff in long WebCore::IDLOperation<WebCore::JSOffscreenCanvasRenderingContext2D>::call<&WebCore::jsOffscreenCanvasRenderingContext2DPrototypeFunction_measureTextBody, (WebCore::CastedThisErrorBehavior)0>(JSC::JSGlobalObject&, JSC::CallFrame&, char const*) () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1. #14 0x00007f99acccf28f in WebCore::jsOffscreenCanvasRenderingContext2DPrototypeFunction_measureText(JSC::JSGlobalObject*, JSC::CallFrame*) () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #15 0x00007f9961d9ca78 in () #16 0x00007f988b1fe180 in () #17 0x00007f99ac1a8da0 in op_call_slow_return_location () at /app/webkit/WebKitBuild/Debug/lib/libWPEWebKit-1.1.so.0 #18 0x0000000000000000 in ()