Disable system fonts on Android (issue 18210)

To avoid introducing any inline "hacks" in the viewer-code this meant adding `useSystemFonts` to the AppOptions, which thus required some new functionality since the default value should be `undefined` given how the option is handled in the API; note [this code](https://github.com/mozilla/pdf.js/blob/ed83d7c5e16798a56c493d56aaa8200dd280bb17/src/display/api.js#L298-L301).

Finally, also moves the definition of the development-mode `window.isGECKOVIEW` property to the HTML file such that it's guaranteed to be set regardless of how and when it's accessed.

Author: Snuffleupagus

web/app.js

@@ -1008,16 +1008,6 @@ const PDFViewerApplication = {
       AppOptions.set("docBaseUrl", this.baseUrl);
     }
 
-    // On Android, there is almost no chance to have the font we want so we
-    // don't use the system fonts in this case.
-    if (
-      typeof PDFJSDev === "undefined"
-        ? window.isGECKOVIEW
-        : PDFJSDev.test("GECKOVIEW")
-    ) {
-      args.useSystemFonts = false;
-    }
-
     // Set the necessary API parameters, using all the available options.
     const apiParams = AppOptions.getAll(OptionKind.API);
     const loadingTask = getDocument({

web/app_options.js

@@ -34,11 +34,19 @@ if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
 
   // Limit canvas size to 5 mega-pixels on mobile.
   // Support: Android, iOS
-  (function checkCanvasSizeLimitation() {
+  (function () {
     if (isIOS || isAndroid) {
       compatibilityParams.maxCanvasPixels = 5242880;
     }
   })();
+
+  // Don't use system fonts on Android (issue 18210).
+  // Support: Android
+  (function () {
+    if (isAndroid) {
+      compatibilityParams.useSystemFonts = false;
+    }
+  })();
 }
 
 const OptionKind = {
@@ -47,6 +55,7 @@ const OptionKind = {
   API: 0x04,
   WORKER: 0x08,
   EVENT_DISPATCH: 0x10,
+  UNDEF_ALLOWED: 0x20,
   PREFERENCE: 0x80,
 };
 
@@ -377,6 +386,19 @@ const defaultOptions = {
           : "../web/standard_fonts/",
     kind: OptionKind.API,
   },
+  useSystemFonts: {
+    // On Android, there is almost no chance to have the font we want so we
+    // don't use the system fonts in this case (bug 1882613).
+    /** @type {boolean|undefined} */
+    value: (
+      typeof PDFJSDev === "undefined"
+        ? window.isGECKOVIEW
+        : PDFJSDev.test("GECKOVIEW")
+    )
+      ? false
+      : undefined,
+    kind: OptionKind.API + OptionKind.UNDEF_ALLOWED,
+  },
   verbosity: {
     /** @type {number} */
     value: 1,
@@ -464,6 +486,11 @@ if (typeof PDFJSDev === "undefined" || PDFJSDev.test("TESTING || LIB")) {
       if (kind & OptionKind.BROWSER) {
         throw new Error(`Cannot mix "PREFERENCE" and "BROWSER" kind: ${name}`);
       }
+      if (kind & OptionKind.UNDEF_ALLOWED) {
+        throw new Error(
+          `Cannot have \`undefined\` value for "PREFERENCE" kind: ${name}`
+        );
+      }
       if (
         typeof compatibilityParams === "object" &&
         compatibilityParams[name] !== undefined
@@ -480,6 +507,12 @@ if (typeof PDFJSDev === "undefined" || PDFJSDev.test("TESTING || LIB")) {
       ) {
         throw new Error(`Invalid value for "PREFERENCE" kind: ${name}`);
       }
+    } else if (kind & OptionKind.BROWSER) {
+      if (kind & OptionKind.UNDEF_ALLOWED) {
+        throw new Error(
+          `Cannot have \`undefined\` value for "BROWSER" kind: ${name}`
+        );
+      }
     }
   }
 }
@@ -513,7 +546,13 @@ class AppOptions {
   static set(name, value) {
     const defaultOption = defaultOptions[name];
 
-    if (!defaultOption || typeof value !== typeof defaultOption.value) {
+    if (
+      !defaultOption ||
+      !(
+        typeof value === typeof defaultOption.value ||
+        defaultOption.kind & OptionKind.UNDEF_ALLOWED
+      )
+    ) {
       return;
     }
     userOptions[name] = value;
@@ -526,7 +565,13 @@ class AppOptions {
       const defaultOption = defaultOptions[name],
         userOption = options[name];
 
-      if (!defaultOption || typeof userOption !== typeof defaultOption.value) {
+      if (
+        !defaultOption ||
+        !(
+          typeof userOption === typeof defaultOption.value ||
+          defaultOption.kind & OptionKind.UNDEF_ALLOWED
+        )
+      ) {
         continue;
       }
       if (prefs) {
@@ -554,9 +599,8 @@ class AppOptions {
 
     if (typeof PDFJSDev === "undefined" || PDFJSDev.test("GENERIC")) {
       // Re-apply a compatibility-value, if it exists, to the user-options.
-      const val = compatibilityParams[name];
-      if (val !== undefined) {
-        userOptions[name] = val;
+      if (name in compatibilityParams) {
+        userOptions[name] = compatibilityParams[name];
       }
     }
   }

web/viewer-geckoview.html

@@ -42,6 +42,12 @@
 <!--#endif-->
 
 <!--#if !MOZCENTRAL-->
+    <script>
+      if (typeof PDFJSDev === "undefined") {
+        window.isGECKOVIEW = true;
+      }
+    </script>
+
     <script type="importmap">
       {
         "imports": {

web/viewer-geckoview.js

@@ -60,9 +60,6 @@ function getViewerConfiguration() {
 function webViewerLoad() {
   const config = getViewerConfiguration();
 
-  if (typeof PDFJSDev === "undefined") {
-    window.isGECKOVIEW = true;
-  }
   PDFViewerApplication.run(config);
 }