[Navigator] Fix Mac UX Issues for Set Desktop Wallpaper

This doesn't make it functional for versions of OS X
Mozilla tracked an issue in Bug #1629313 citing OS X 10.14+. So this busted for those versions on all UXP applications until this is worked around/fixed/mangled.
More data on exact OS Versioning is need as well as a full on Mac developer.

navigator/comm/content/nsContextMenu.js

@@ -948,8 +948,27 @@ nsContextMenu.prototype = {
   },
 
   setDesktopBackground: function() {
-    openDialog("chrome://navigator/content/setDesktopBackground.xul",
-               "_blank", "chrome,modal,titlebar,centerscreen", this.target);
+    const kDesktopBackgroundURL = "chrome://navigator/content/setDesktopBackground.xul";
+#ifdef XP_MACOSX
+    // On Mac, the Set Desktop Background window is not modal.
+    // Don't open more than one Set Desktop Background window.
+    var wm = Components.classes["@mozilla.org/appshell/window-mediator;1"]
+                       .getService(Components.interfaces.nsIWindowMediator);
+    var dbWin = wm.getMostRecentWindow("Shell:SetDesktopBackground");
+    if (dbWin) {
+      dbWin.gSetBackground.init(this.target);
+      dbWin.focus();
+    } else {
+      openDialog(kDesktopBackgroundURL, "",
+                 "centerscreen,chrome,dialog=no,dependent,resizable=no",
+                 this.target);
+    }
+#else
+    // On non-Mac platforms, the Set Wallpaper dialog is modal.
+    openDialog(kDesktopBackgroundURL, "",
+               "centerscreen,chrome,dialog,modal,dependent",
+               this.target);
+#endif
   },
 
   // Save URL of clicked-on frame.

navigator/comm/jar.mn

@@ -30,7 +30,7 @@ comm.jar:
   content/communicator/findUtils.js                                (content/findUtils.js)
   content/communicator/fullscreen-video.xhtml                      (content/fullscreen-video.xhtml)
   content/communicator/gopherAddon.xhtml                           (content/gopherAddon.xhtml)
-  content/communicator/nsContextMenu.js                            (content/nsContextMenu.js)
+* content/communicator/nsContextMenu.js                            (content/nsContextMenu.js)
   content/communicator/openLocation.js                             (content/openLocation.js)
   content/communicator/openLocation.xul                            (content/openLocation.xul)
   content/communicator/safeMode.js                                 (content/safeMode.js)

navigator/components/shell/content/setDesktopBackground.xul

@@ -51,17 +51,8 @@
         <image id="monitor"/>
       </stack>
     </vbox>
-
+#ifndef XP_MACOSX
     <vbox flex="1">
-#ifdef XP_MACOSX
-      <button id="setDesktopBackground"
-              label="&setDesktopBackground.title;" 
-              oncommand="gSetBackground.setDesktopBackground();"/>
-      <button id="showDesktopPreferences"
-              label="&openDesktopPrefs.label;"
-              oncommand="gSetBackground.showDesktopPrefs();"
-              hidden="true"/>
-#else
       <hbox align="center">
         <label value="&color.label;"/>
         <spacer class="colorSpacer"/>
@@ -84,9 +75,20 @@
           </menupopup>
         </menulist>
       </hbox>
-#endif
     </vbox>
+#endif
   </hbox>
 
   <separator class="groove"/>
+#ifdef XP_MACOSX
+  <hbox align="right">
+    <button id="setDesktopBackground"
+            label="&setDesktopBackground.title;"
+            oncommand="gSetBackground.setDesktopBackground();"/>
+    <button id="showDesktopPreferences"
+            label="&openDesktopPrefs.label;"
+            oncommand="gSetBackground.showDesktopPrefs();"
+            hidden="true"/>
+  </hbox>
+#endif
 </dialog>