CompositableHost.cpp patch added by @q66, based on patch in issue 1602730.

# HG changeset patch
# Parent  46ea866ca3acb8bb5e1709ceb799b9c94f591dec
Problem description: Tab-titles that are too long to fit into a tab get faded out.
                     On big endian this is broken and instead of fading out, the 
                     tab gets white and the font transparent, leading to an unreadable
                     tab-title
Solution: This is not a real solution, but a hack. The real solution would have been
          to byte-swap the correct buffer, but I could not find it.
          So the next best thing is to deactivate the fading-effect. Now all tab-titles
          are readable, albeit not as pretty to look at as they could be. 
Side-effects: I have not yet found an unwanted side-effect.

diff -r 46ea866ca3ac -r 6ef20eee3f8f gfx/2d/DrawTargetSkia.cpp
--- a/gfx/2d/DrawTargetSkia.cpp	Tue Oct 22 12:27:22 2019 +0200
+++ b/gfx/2d/DrawTargetSkia.cpp	Thu Oct 31 09:11:56 2019 +0100
@@ -1861,6 +1861,14 @@
       SkCanvas::kPreserveLCDText_SaveLayerFlag |
           (aCopyBackground ? SkCanvas::kInitWithPrevious_SaveLayerFlag : 0));
 
+#if MOZ_BIG_ENDIAN()
+  // Pushing a layer where an aMask is defined produces wrong output.
+  // We _should_ endian swap the data, but I couldn't find a workable way to do so
+  // Therefore I deactivate those layers in the meantime.
+  // The result is: Tab-titles that are longer than the available space should be faded out.
+  //                The fading doesn't work, so we deactivate the fading-effect here.
+  if (!aMask)
+#endif
   mCanvas->saveLayer(saveRec);
 
   SetPermitSubpixelAA(aOpaque);
--- a/gfx/layers/composite/CompositableHost.cpp
+++ b/gfx/layers/composite/CompositableHost.cpp
@@ -13,6 +13,7 @@
 #include "Effects.h"  // for EffectMask, Effect, etc
 #include "gfxUtils.h"
 #include "Layers.h"
+#include "mozilla/EndianUtils.h"
 #include "mozilla/gfx/gfxVars.h"
 #include "mozilla/layers/LayersSurfaces.h"  // for SurfaceDescriptor
 #include "mozilla/layers/TextureHost.h"     // for TextureHost, etc
@@ -92,9 +93,13 @@ bool CompositableHost::AddMaskEffect(EffectChain& aEffects,
   }
   MOZ_ASSERT(source);
 
+  // Setting an alpha-mask here breaks the URL-bar on big endian (s390x)
+  // if the typed URL is too long for the textbox (automatic scrolling needed)
+#if MOZ_LITTLE_ENDIAN()
   RefPtr<EffectMask> effect =
       new EffectMask(source, source->GetSize(), aTransform);
   aEffects.mSecondaryEffects[EffectTypes::MASK] = effect;
+#endif
   return true;
 }
 
