fix: handle malformed PDFs and checkbox appearance states during flatten

- Use lookupMaybe() instead of lookup() for FT attribute to gracefully
  handle malformed PDFs with missing or invalid field types
  Hopding/pdf-lib#1519

- Use widget's appearance state (/AS) when available instead of relying
  solely on field value (/V) to find correct appearance in /AP/N dict
  Hopding/pdf-lib#1574

Coded by an LLM.

Author: folknor

src/api/form/PDFForm.ts

@@ -791,8 +791,17 @@ export default class PDFForm {
         refOrDict = lookedUp;
       }
       if (refOrDict instanceof PDFDict) {
-        const value = field.acroField.getValue();
-        const ref = refOrDict.get(value) ?? refOrDict.get(PDFName.of('Off'));
+        // Use the widget's appearance state (/AS) if available, otherwise
+        // fall back to the field value (/V). The /AS value is the key in
+        // the /AP/N dict that specifies which appearance to show.
+        // This fixes #1574 where checkmarks disappeared because /V and /AS
+        // might have different values (e.g., /V=/Yes but /AS=/1)
+        const appearanceState = widget.getAppearanceState();
+        const fieldValue = field.acroField.getValue();
+        const ref =
+          (appearanceState && refOrDict.get(appearanceState)) ??
+          refOrDict.get(fieldValue) ??
+          refOrDict.get(PDFName.of('Off'));
 
         if (ref instanceof PDFRef) {
           refOrDict = ref;

src/core/acroform/utils.ts

@@ -77,7 +77,9 @@ const isNonTerminalAcroField = (dict: PDFDict): boolean => {
 
 const createPDFAcroTerminal = (dict: PDFDict, ref: PDFRef): PDFAcroTerminal => {
   const ftNameOrRef = getInheritableAttribute(dict, PDFName.of('FT'));
-  const type = dict.context.lookup(ftNameOrRef, PDFName);
+  // Use lookupMaybe to handle malformed PDFs where FT is missing or invalid
+  // This fixes crashes with certain government forms (#1519)
+  const type = dict.context.lookupMaybe(ftNameOrRef, PDFName);
 
   if (type === PDFName.of('Btn')) return createPDFAcroButton(dict, ref);
   if (type === PDFName.of('Ch')) return createPDFAcroChoice(dict, ref);