--- a/image/src/Decoder.cpp
+++ b/image/src/Decoder.cpp
@@ -189,16 +189,19 @@ Decoder::Finish(ShutdownReason aReason)
       if (GetCompleteFrameCount() == 0) {
         usable = false;
       }
     }
 
     // If we're usable, do exactly what we should have when the decoder
     // completed.
     if (usable) {
+      // Not writing to the entire frame may have left us transparent.
+      PostHasTransparency();
+
       if (mInFrame) {
         PostFrameStop();
       }
       PostDecodeDone();
     } else {
       if (!IsSizeDecode()) {
         mProgress |= FLAG_DECODE_COMPLETE | FLAG_ONLOAD_UNBLOCKED;
       }