Bug 1103328 (Part 1) - Fire HAS_TRANSPARENCY for clear disposal methods. r=tn

image/decoders/nsGIFDecoder2.cpp

@@ -828,6 +828,18 @@ nsGIFDecoder2::WriteInternal(const char* aBuffer, uint32_t aCount,
       if (mGIFStruct.disposal_method == 4) {
         mGIFStruct.disposal_method = 3;
       }
+
+      {
+        int32_t method =
+          FrameBlender::FrameDisposalMethod(mGIFStruct.disposal_method);
+        if (method == FrameBlender::kDisposeClearAll ||
+            method == FrameBlender::kDisposeClear) {
+          // We may have to display the background under this image during
+          // animation playback, so we regard it as transparent.
+          PostHasTransparency();
+        }
+      }
+
       mGIFStruct.delay_time = GETINT16(q + 1) * 10;
       GETN(1, gif_consume_block);
       break;

image/decoders/nsPNGDecoder.cpp

@@ -176,6 +176,12 @@ void nsPNGDecoder::CreateFrame(png_uint_32 x_offset, png_uint_32 y_offset,
 #ifdef PNG_APNG_SUPPORTED
   if (png_get_valid(mPNG, mInfo, PNG_INFO_acTL)) {
     mAnimInfo = AnimFrameInfo(mPNG, mInfo);
+
+    if (mAnimInfo.mDispose == FrameBlender::kDisposeClear) {
+      // We may have to display the background under this image during
+      // animation playback, so we regard it as transparent.
+      PostHasTransparency();
+    }
   }
 #endif
 }