fix preference pane count tab reported by screen readers via FireFox

src/gwt/src/org/rstudio/core/client/prefs/SectionChooser.java

@@ -37,6 +37,9 @@
 import org.rstudio.core.client.a11y.A11y;
 import org.rstudio.core.client.widget.DecorativeImage;
 
+import java.util.LinkedHashSet;
+import java.util.Set;
+
 /**
  * Vertical tab control used by Preferences dialogs. Follows the ARIA tab pattern.
  * 
@@ -128,6 +131,13 @@ public Id addSection(ImageResource icon, String name)
  Roles.getTabRole().setAriaSelectedState(panel.getElement(), SelectedValue.FALSE);
  Roles.getTabRole().setAriaControlsProperty(panel.getElement(), getTabPanelId(sectionTabId));
  inner_.add(panel);
+
+ // FireFox fails to enumerate the tabs when building its accessibility tree, 
+ // perhaps due to the deep nesting of layout tables. Use the aria-owns attribute 
+ // to assist it. https://github.com/rstudio/rstudio/issues/5120
+ tabIds_.add(sectionTabId);
+ Roles.getTablistRole().setAriaOwnsProperty(getElement(), tabIds_.toArray(new Id[0]));
+
  return sectionTabId;
  }
 
@@ -274,6 +284,6 @@ private int sectionCount()
 
  private Integer selectedIndex_;
  private final VerticalPanel inner_ = new VerticalPanel();
- private static final PreferencesDialogBaseResources res_ = 
-  PreferencesDialogBaseResources.INSTANCE;
+ private static final PreferencesDialogBaseResources res_ = PreferencesDialogBaseResources.INSTANCE;
+ private Set<Id> tabIds_ = new LinkedHashSet<>();
 }