How to keep the bottom menu irrespective of the layouts above this is visible or not

I have many horizontal layouts in my screen. I'm using Popup extension to display the layouts according to the selection, particular layout elements are displayed
Popup values are
About
News
Events
Gallery
screen initialize, only HL1 and HL5 should be displayed
HL1 - (About) By default, this is displayed
HL2 - (News) if News is selected, Hl1, Hl3 and HL4 hidden
HL3- (Events) if Events is selected, HL1,HL2 and HL4 hidden
HL4(Gallery) if Gallery is selected, HL1,HL2 and HL3 hidden
HL5 (bottom menu) should be visible and always at the bottom

The issue is, only on screen initialize, HL5 is at the correct position (because the About content fills the screen)

rest of the options doesn't fill the screen, so HL5 goes for a toss

irrespective of the menus visible or not, how to place the bottom menu fixed at the bottom of the screen