Solution:
This is a bug which crops in when within the layout, you set the parent container's overflow property to “auto” and placing a relatively positioned item within it. This relatively positioned item tends to violate the parent element's boundaries and overflows. The simplest fix to this bug is to position the parent container relatively.
Since it was not really possible to change the positioning of the menu, I removed the button's tooltip and instead used CSS to create a DIV element which behaved exactly like a tooltip, containing the original tooltip's text. That way, I could control the element's position and ensure no elements are hidden. The element was also styled to resemble a typical browser tooltip.
Using flexboxes for this purpose is a good solution, however, further css must be implemented in order for the content to be responsive.
An alternative would be to use a wrapper containing both items, and giving it a "position:relative". Then using "position: absolute" in both of your divs (assuming you've got one for the bar on top and anotherone for the content). You would also need to restrict the heights of the top-bar component.
FOR EXAMPLE:
#wrapper{
position: relative;
}
#top-bar, #content{
position: absolute;
}
#top-bar{
height: 50px;
width: 100%;
}
#content{
width:100%;
}