z-index used for different elements:

Element z-index
Gallery images 1
Buttons 2
Modal Windows & Right Panels 4, 5, -1
Applications Catalogue 5
Form Tooltips 9, 10, 11, -11
Gallery slideshow 100
Element menu 1, 4, 998, -1
Right Panel - Inline editor 998
Text & Image - alert message 1000
Arrangement UI placeholders 1000
Content Placeholders 1000, -1
Left Menu 1200
Top Menu 1250
Design Selector 5000
Autocomplete (tooltips) 1, 10, 8800
Scrollbar 8900
Lightbox 9000
DatePicker 9004
Progress UI 6, 10000