Fix wonky webtoon layout on less faster devices

(cherry picked from commit 0d28fb31a39c92f19e8cb8fc0ecf5ca1523c1085)

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/webtoon/WebtoonPageHolder.kt

@@ -323,6 +323,13 @@ class WebtoonPageHolder(
         initRetryLayout().isVisible = true
     }
 
+    /**
+     * Called when the image is decoded and going to be displayed.
+     */
+    private fun onImageDecoded() {
+        progressContainer.isVisible = false
+    }
+
     /**
      * Called when the image fails to decode.
      */
@@ -376,6 +383,10 @@ class WebtoonPageHolder(
             setCropBorders(cropBorders)
             setOnImageEventListener(
                 object : SubsamplingScaleImageView.DefaultOnImageEventListener() {
+                    override fun onReady() {
+                        onImageDecoded()
+                    }
+
                     override fun onImageLoadError(e: Exception) {
                         onImageDecodeError()
                     }
@@ -507,6 +518,7 @@ class WebtoonPageHolder(
                         result.start()
                     }
                     setImageDrawable(result)
+                    onImageDecoded()
                 },
                 onError = {
                     onImageDecodeError()