Implement accessibilityPlaceholderValue.

native/Avalonia.Native/src/OSX/automation.mm

@@ -438,6 +438,13 @@ private:
     return [super accessibilityNumberOfCharacters];
 }
 
+- (NSString *)accessibilityPlaceholderValue
+{
+    if (_peer->IsTextProvider())
+        return GetNSStringAndRelease(_peer->TextProvider_GetPlaceholderText());
+    return [super accessibilityPlaceholderValue];
+}
+
 - (NSRange)accessibilityRangeForLine:(NSInteger)line
 {
     if (_peer->IsTextProvider())

src/Avalonia.Native/AvnAutomationPeer.cs

@@ -160,6 +160,8 @@ namespace Avalonia.Native
             *start = range.Start;
             *length = range.Length;
         }
+
+        public IAvnString TextProvider_GetPlaceholderText() => ((ITextProvider)_inner).PlaceholderText.ToAvnString();
         
         public IAvnString TextProvider_GetSelectedText()
         {

src/Avalonia.Native/avn.idl

@@ -919,6 +919,7 @@ interface IAvnAutomationPeer : IUnknown
      int TextProvider_GetCaretLineNumber();
      int TextProvider_GetLineForIndex(int index);
      void TextProvider_GetLineRange(int lineIndex, int* start, int* length);
+     IAvnString* TextProvider_GetPlaceholderText();
      IAvnString* TextProvider_GetText(int start, int length);
      int TextProvider_GetTextLength();
      IAvnString* TextProvider_GetSelectedText();