Cherry-pick 259548.8@safari-7615-branch (5a0f792). https://bugs.webki…

…t.org/show_bug.cgi?id=251158

    Check color opacity after lossy conversion when blending
    https://bugs.webkit.org/show_bug.cgi?id=251158
    rdar://104553839

    Reviewed by Dean Jackson.

    We check opacity to determine if we should forgo blending,
    however after performing a lossy conversion we can end
    up with alpha values that result in a division by zero.
    Add an additional check after conversion to prevent this
    case.

    * LayoutTests/fast/backgrounds/background-color-lch-crash-expected.txt: Added.
    * LayoutTests/fast/backgrounds/background-color-lch-crash.html: Added.
    * Source/WebCore/platform/graphics/ColorBlending.cpp:
    (WebCore::blendSourceOver):

    Canonical link: https://commits.webkit.org/259548.8@safari-7615-branch

LayoutTests/fast/backgrounds/background-color-lch-crash-expected.txt

@@ -0,0 +1 @@
+This test passes if it does not crash

LayoutTests/fast/backgrounds/background-color-lch-crash.html

@@ -0,0 +1,14 @@
+<style>
+    html, body {
+      background-color: lch(0 0 0 / 0.001);
+    }
+  </style>
+  <script>
+    internals.settings.setCaretBrowsingEnabled(true);
+    onload = () => {
+      document.execCommand('SelectAll');
+    };
+    if (window.testRunner)
+        testRunner.dumpAsText();
+  </script>
+  <span>This test passes if it does not crash</span>

Source/WebCore/platform/graphics/ColorBlending.cpp

@@ -42,6 +42,12 @@ Color blendSourceOver(const Color& backdrop, const Color& source)
     auto [backdropR, backdropG, backdropB, backdropA] = backdrop.toColorTypeLossy<SRGBA<uint8_t>>().resolved();
     auto [sourceR, sourceG, sourceB, sourceA] = source.toColorTypeLossy<SRGBA<uint8_t>>().resolved();
 
+    if (!backdropA || sourceA == 255)
+        return source;
+
+    if (!sourceA)
+        return backdrop;
+
     int d = 0xFF * (backdropA + sourceA) - backdropA * sourceA;
     int a = d / 0xFF;
     int r = (backdropR * backdropA * (0xFF - sourceA) + 0xFF * sourceA * sourceR) / d;