Steps to recreate:

1. Open the activity chooser dialogue 
2. Resize the window so that the height is smaller than the activity chooser. 

Result: The buttons disappears below the window.  (In the classic clean theme the buttons are still display in the dialogue. 

Cause: 

In 
moodle/lib/yui/src/chooserdialogue/js/chooserdialogue.js

the heigh is calculated with 

/ Take off 15px top and bottom for borders, plus 40px each for the title and button area before setting the
{{ // new max-height}}
{{ totalheight = newheight;}}
{{ newheight = newheight - (15 + 15 + 40 + 40);}}
{{ dialogue.setStyle('maxHeight', newheight + 'px');}}

But in Boost themes the titel and button area is 69 + 57 . 

Ideally the values should be fetched dynamically but otherwise at least adjusted to match the boost values.