Additional changes from requests in PR #640

apps/avifdec.c

diff --git a/apps/avifdec.c b/apps/avifdec.c
index 8a90ff3..c60bd53 100644
--- a/apps/avifdec.c
+++ b/apps/avifdec.c
@@ -37,8 +37,8 @@
     printf("    -r,--raw-color    : Output raw RGB values instead of multiplying by alpha when saving to opaque formats\n");
     printf("                        (JPEG only; not applicable to y4m)\n");
     printf("    --index           : When decoding an image sequence or progressive image, specify which frame index to decode (Default: 0)\n");
-    printf("    --progressive     : Enable progressive AVIF processing. If a progressive image is encountered, avifdec will use\n");
-    printf("                        --index to choose which layer to decode (in progressive order).\n");
+    printf("    --progressive     : Enable progressive AVIF processing. If a progressive image is encountered and --progressive is passed,\n");
+    printf("                        avifdec will use --index to choose which layer to decode (in progressive order).\n");
     printf("    --no-strict       : Disable strict decoding, which disables strict validation checks and errors\n");
     printf("    -i,--info         : Decode all frames and display all image information instead of saving to disk\n");
     printf("    --ignore-icc      : If the input file contains an embedded ICC profile, ignore it (no-op if absent)\n");

src/read.c

diff --git a/src/read.c b/src/read.c
index 7be9f1f..e07e2df 100644
--- a/src/read.c
+++ b/src/read.c
@@ -554,7 +554,7 @@
         sample->size = sampleSize;
         sample->spatialID = (uint8_t)lselProp->u.lsel.layerID;
         sample->sync = AVIF_TRUE;
-    } else if (allowProgressive && a1lxProp) {
+    } else if (allowProgressive && item->progressive) {
         // Progressive image. Decode all layers and expose them all to the user.
 
         if (imageCountLimit && (layerCount > imageCountLimit)) {
@@ -954,7 +954,12 @@
 {
     if (item->mergedExtents.data && !item->partialMergedExtents) {
         // Multiple extents have already been concatenated for this item, just return it
-        memcpy(outData, &item->mergedExtents, sizeof(avifROData));
+        if (offset >= item->mergedExtents.size) {
+            avifDiagnosticsPrintf(diag, "Item ID %u read has overflowing offset", item->id);
+            return AVIF_RESULT_TRUNCATED_DATA;
+        }
+        outData->data = item->mergedExtents.data + offset;
+        outData->size = item->mergedExtents.size - offset;
         return AVIF_RESULT_OK;
     }