7904082: The UI for API Diff could be improved

src/share/classes/jdk/codetools/apidiff/Options.java

@@ -171,6 +171,7 @@ public String toString() {
     Boolean compareDocComments;
     Boolean compareApiDescriptions;
     Boolean compareApiDescriptionsAsText;
+    Boolean showUnchanged;
     String jdkDocs;
 
     // output options
@@ -181,7 +182,6 @@ public String toString() {
     Path mainStylesheet;
     List<Path> extraStylesheets;
     List<Path> resourceFiles;
-    boolean showEqual;
 
     /**
      * The position of additional text to be included in the report.
@@ -336,6 +336,16 @@ void process(String opt, String arg, Options options) throws BadOption {
             }
         },
 
+        /**
+         * {@code --show-unchanged} <var>boolean-value</var>
+         */
+        SHOW_UNCHANGED("--show-unchanged", "opt.arg.boolean") {
+            @Override
+            void process(String opt, String arg, Options options) throws BadOption {
+                options.showUnchanged = asBoolean(arg);
+            }
+        },
+
         /**
          * {@code --enable-preview}.
          *
@@ -909,6 +919,15 @@ public boolean compareApiDescriptionsAsText() {
         return compareApiDescriptionsAsText;
     }
 
+    /**
+     * Returns whether unchanged API elements should be unconditionally shown.
+     *
+     * @return {@code true} if unchanged API elements should be unconditionally shown
+     */
+    public boolean showUnchanged() {
+        return showUnchanged;
+    }
+
     /**
      * Returns whether documentation comments should be compared.
      *
@@ -981,10 +1000,6 @@ public List<Path> getResourceFiles() {
         return (resourceFiles == null) ? Collections.emptyList() : resourceFiles;
     }
 
-    public boolean showEqual() {
-        return showEqual;
-    }
-
     /**
      * Returns the value of a "hidden" option, set by {@code -XD<name>} or
      * {@code -XD<name>=<value>}.
@@ -1082,6 +1097,10 @@ void validate() {
             compareDocComments = !compareApiDescriptions;
         }
 
+        if (showUnchanged == null) {
+            showUnchanged = false;
+        }
+
         if (resourceFiles != null) {
             for (var resFile : resourceFiles) {
                 if (resFile.isAbsolute() && !Files.exists(resFile)) {

src/share/classes/jdk/codetools/apidiff/report/html/PageReporter.java

@@ -513,7 +513,7 @@ protected Content buildNav(InfoTextKind kind) {
         }
         List<Content> infoBox = new ArrayList<>();
         infoBox.add(new RawHtml(infoText));
-        if (kind == InfoTextKind.HEADER) {
+        if (kind == InfoTextKind.HEADER && !parent.options.showUnchanged()) {
             String showUnchangedCheckbox =
                     """
                     <script>

src/share/classes/jdk/codetools/apidiff/resources/help.properties

@@ -127,6 +127,9 @@ opt.desc.release=\
 opt.desc.resource-files=\
     Specifies resource files to be copied from an API directory
 
+opt.desc.show-unchanged=\
+    Unchanged elements should be unconditionally shown in the resulting diff
+
 opt.desc.source=\
     Specifies the version of the platform for an API.
 

src/share/doc/apidiff.md

@@ -287,6 +287,11 @@ to repeat these options for each API to be compared.
     <p class="note">This option may be useful when comparing HTML documentation that
     depend on some non-HTML resource files.</p>
 
+<a id="option-show-unchanged">`--show-unchanged` *boolean*</a>
+:   If true, unchanged elements will be show unconditionally. When false or
+    missing, the user viewing the diff will have an option to show or hide the
+    unchanged elements.
+
 ### Other Options
 
 <a id="option-help">`--help`, `-help`, `-h`, `-?`</a>