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diff -r 6ef20eee3f8f gfx/layers/basic/BasicCompositor.cpp
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--- a/gfx/layers/basic/BasicCompositor.cpp	Thu Oct 31 09:11:56 2019 +0100
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+++ b/gfx/layers/basic/BasicCompositor.cpp	Wed Dec 11 16:16:09 2019 +0100
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@@ -693,9 +693,13 @@
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   RefPtr<SourceSurface> sourceMask;
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   Matrix maskTransform;
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+  // Setting an alpha-mask here breaks the URL-bar on big endian (s390x)
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+  // if the typed URL is too long for the textbox (automatic scrolling needed)
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+#if MOZ_LITTLE_ENDIAN
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   if (aTransform.Is2D()) {
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     SetupMask(aEffectChain, dest, offset, sourceMask, maskTransform);
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   }
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+#endif
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   CompositionOp blendMode = CompositionOp::OP_OVER;
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   if (Effect* effect =
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diff -r 6ef20eee3f8f gfx/layers/composite/CompositableHost.cpp
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--- a/gfx/layers/composite/CompositableHost.cpp	Thu Oct 31 09:11:56 2019 +0100
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+++ b/gfx/layers/composite/CompositableHost.cpp	Wed Dec 11 16:16:09 2019 +0100
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@@ -91,6 +91,7 @@
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   }
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   MOZ_ASSERT(source);
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+  // Alternatively: Comment out these lines where the alpha-mask is set
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   RefPtr<EffectMask> effect =
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       new EffectMask(source, source->GetSize(), aTransform);
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   aEffects.mSecondaryEffects[EffectTypes::MASK] = effect;