Fix incorrect progress when resuming resumable image downloads (#3616)

core/common/src/main/kotlin/eu/kanade/tachiyomi/network/OkHttpExtensions.kt

@@ -136,8 +136,9 @@ fun OkHttpClient.newCachelessCallWithProgress(
                 .build()
 
             val originalResponse = chain.proceed(request)
+            val actualExistingSize = if (originalResponse.code == 206) existingSize else 0L
             originalResponse.newBuilder()
-                .body(ProgressResponseBody(originalResponse.body, listener, existingSize))
+                .body(ProgressResponseBody(originalResponse.body, listener, actualExistingSize))
                 .build()
         }
         .build()

core/common/src/main/kotlin/eu/kanade/tachiyomi/network/ProgressResponseBody.kt

@@ -40,9 +40,15 @@ class ProgressResponseBody(
                 val bytesRead = super.read(sink, byteCount)
                 // read() returns the number of bytes read, or -1 if this source is exhausted.
                 totalBytesRead += if (bytesRead != -1L) bytesRead else 0
+
+                // contentLength() returns -1L if Content-Length is missing
+                val totalLength = responseBody.contentLength().let {
+                    if (it != -1L) it + existingSize else -1L
+                }
+
                 progressListener.update(
                     totalBytesRead,
-                    responseBody.contentLength(),
+                    totalLength,
                     bytesRead == -1L,
                 )
                 return bytesRead