Update the current stride before composing when decoding a text region

Fixes #17871.

We do something similar to:
https://source.chromium.org/chromium/chromium/src/+/main:third_party/pdfium/core/fxcodec/jbig2/JBig2_TrdProc.cpp;l=373-379;drc=24c6be6924df3ff585bb63f6aed4e2c81e791fb2

src/core/jbig2.js

@@ -865,6 +865,20 @@ function decodeTextRegion(
           decodingContext
         );
       }
+
+      let increment = 0;
+      if (!transposed) {
+        if (referenceCorner > 1) {
+          currentS += symbolWidth - 1;
+        } else {
+          increment = symbolWidth - 1;
+        }
+      } else if (!(referenceCorner & 1)) {
+        currentS += symbolHeight - 1;
+      } else {
+        increment = symbolHeight - 1;
+      }
+
       const offsetT = t - (referenceCorner & 1 ? 0 : symbolHeight - 1);
       const offsetS = currentS - (referenceCorner & 2 ? symbolWidth - 1 : 0);
       let s2, t2, symbolRow;
@@ -896,7 +910,6 @@ function decodeTextRegion(
               );
           }
         }
-        currentS += symbolHeight - 1;
       } else {
         for (t2 = 0; t2 < symbolHeight; t2++) {
           row = bitmap[offsetT + t2];
@@ -921,7 +934,6 @@ function decodeTextRegion(
               );
           }
         }
-        currentS += symbolWidth - 1;
       }
       i++;
       const deltaS = huffman
@@ -930,7 +942,7 @@ function decodeTextRegion(
       if (deltaS === null) {
         break; // OOB
       }
-      currentS += deltaS + dsOffset;
+      currentS += increment + deltaS + dsOffset;
     } while (true);
   }
   return bitmap;

test/pdfs/.gitignore

@@ -637,3 +637,5 @@
 !issue17730.pdf
 !bug1883609.pdf
 !issue17808.pdf
+!issue17871_bottom_right.pdf
+!issue17871_top_right.pdf

test/test_manifest.json

@@ -9801,5 +9801,19 @@
     "md5": "dd66194bdaa0f62bc96c760384f2e098",
     "rounds": 1,
     "type": "eq"
+  },
+  {
+    "id": "issue17871_bottom_right",
+    "file": "pdfs/issue17871_bottom_right.pdf",
+    "md5": "664807166c5279224abfbf259f701958",
+    "rounds": 1,
+    "type": "eq"
+  },
+  {
+    "id": "issue17871_top_right",
+    "file": "pdfs/issue17871_top_right.pdf",
+    "md5": "07d95eabf18aee0cf38f09eb7b1390ec",
+    "rounds": 1,
+    "type": "eq"
   }
 ]