I added a pdf macro with some content. Then I added a gallery macro below the pdf macro. If I open the gallery the gallery dialog is shown under the pdf macro.

+------+
|PDF   |
|      |-----+
+------+     |
   | Gallery |  <- this is the active gallery dialog
   |         |
   +---------|

I think the z-index of the gallery dialog should be higher than the not active pdf macro. The z-index should be given automatically because I don't know which other overlays with other macros are possible.