Work-around for HighlightAnnotations without a top-level /ExtGState-entry (issue 13242)

For HighlightAnnotations with a built-in appearance stream, we still rely on it to specify the opacity correctly via a suitable blend mode. However, if the Annotation-drawing operators are placed *within* a /XObject of the /Form-type, the /ExtGState won't apply to the final rendering and the result is that the highlighting obscures the underlying text.

The more *correct* and general solution would likely be to somehow modify the implementation in `src/display/canvas.js`, to special-case handling of /Form-type /XObjects when rendering Annotations. Since we can very easily work-around this problem for now by using the "no appearance stream" code-path, doing *something* here ought to be preferable.

This patch is (obviously) merely a work-around, but given that the referenced issue is (as far as I know) the first case we've seen of this problem a simple solution will hopefully suffice for now.

src/core/annotation.js

@@ -2676,7 +2676,17 @@ class HighlightAnnotation extends MarkupAnnotation {
       null
     ));
     if (quadPoints) {
-      if (!this.appearance) {
+      const resources =
+        this.appearance && this.appearance.dict.get("Resources");
+
+      if (!this.appearance || !(resources && resources.has("ExtGState"))) {
+        if (this.appearance) {
+          // Workaround for cases where there's no /ExtGState-entry directly
+          // available, e.g. when the appearance stream contains a /XObject of
+          // the /Form-type, since that causes the highlighting to completely
+          // obsure the PDF content below it (fixes issue13242.pdf).
+          warn("HighlightAnnotation - ignoring built-in appearance stream.");
+        }
         // Default color is yellow in Acrobat Reader
         const fillColor = this.color
           ? Array.from(this.color).map(c => c / 255)

test/pdfs/.gitignore

@@ -254,6 +254,7 @@
 !find_all.pdf
 !helloworld-bad.pdf
 !zerowidthline.pdf
+!issue13242.pdf
 !js-colors.pdf
 !annotation-line-without-appearance-empty-Rect.pdf
 !issue12841_reduced.pdf

test/test_manifest.json

@@ -2324,6 +2324,13 @@
        "lastPage": 1,
        "type": "eq"
     },
+    {  "id": "issue13242",
+       "file": "pdfs/issue13242.pdf",
+       "md5": "ceb2da908c972087eb90e6bae240bce9",
+       "rounds": 1,
+       "type": "eq",
+       "annotations": true
+    },
     {  "id": "issue7229",
        "file": "pdfs/issue7229.pdf",
        "md5": "480e51aae0ac271780e4603d1561d15e",