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CSS overflow: auto by 'touching' bottom div

I want to make a div scrollable when its touching the bottom div.
I tried this:
margin-bottom:30px;
position: relative;
overflow: auto;
but it didn't work.
I created a fiddle tho show you my problem:
https://jsfiddle.net/wp3wvuj2/1/
For explanation: When you type in in a input field a new field is added to the div (This function is simplified). I want that before the input fields touch the element below (the START-div) it gets scrollable (overflow: auto).
Somebody have an idea?
Edit:
I noticed that nobody understands my problem.
I'll try to explain it better:
I have list where players add their names. The list has minimum 4 players maximum ∞.
The start buttonis placed at the bottom. The problem is in a iphone 5 it looks like this:
And now if i would add another player input field it would Overlap with the START-Button. Thats the reason why I want it scrollable now. I already get that work with a fixed height, but i want it responsive!
Because on a iPad for example it looks like this:
And I want prevent an overlap with the start button like this:
So it should get scrollable before it overlaps (dependent on the display size).

Updated JS fiddle, try this, i have updated CSS part in your code
https://jsfiddle.net/wp3wvuj2/2/
.main_input {
width: 209px;
top: 70px;
margin: auto;
margin-bottom:30px;
/* position: relative */
overflow: auto;
height:216px; //Give some height always to apply overflow auto
}
.main_start {
width: 100%;
margin: auto;
/* position: absolute */ //Not required
bottom: 20px;
font-family: Pamela;
font-size: 36px;
text-align: center;
}

I've only changed the styles on class main-input
.main_input {
width: 226px;
height: 234px;
top: 70px;
margin: auto;
margin-bottom:30px;
position: relative;
overflow-y: auto;
}
EDIT:
Please note for this solution to be able to work, I needed to remove the Top and Bottom positions of some elements as they were breaking the layout. Please use Margins or Paddings to get that styling you desire.
This now works to scroll once the space runs out on the page.
https://jsfiddle.net/wp3wvuj2/5/

Slide up a "fixed height div" that splits a web page horizontally?

I need a box that slides up from the bottom of my page. I will use the box to show important information to new users. So for example, immediately after signup, the box will slide up with a welcome message.
I've made this jsfiddle that to some extend exemplifies the desired behaviour. It's just a div that gets slided up from the bottom:
$('.foot').addClass('slide-up', 500, 'easeOutBounce');
However, the code is only to exemplify, because the implementation is insufficient for the following reasons:
The bottom box has a pre-determined 500px height, because it's initially hidden 500px below the browser. Instead, I need just the box height to fit its content. The content will vary, and will even be changed through javascript once loaded.
The bottom box emerges on top of other elements. Instead, I want to split the screen in 2. A bottom half that has as much height as the box content needs. And a top half that behaves just like a regular web page, i.e. if there is too much content the user can just scroll down. To exemplify the described effect you can check this jsfiddle (the code has no relevance though)
How could achieve the described behaviour?

After experimenting with several methods, I ended up with a solution that combines some ideas given in freedomm-n's comments (modify the size of the main div) and in Nikhil's answer (use a flex container). You can see the result in this jsfiddle.
For the following markup:
<div id="divContainer">
<div id="divTop">
Main content
</div>
<div id="divFooter">
Footer content
</div>
</div>
And these styles:
html, body, form
{
margin: 0;
padding: 0;
overflow: hidden;
height: 100%;
}
#divContainer
{
width: 100%;
height: 100%;
}
#divTop
{
overflow-y: auto;
padding: 8px;
height: calc(100vh - 16px); /* Accounts for padding and border (if any) */
}
#divFooter
{
padding: 12px;
text-align: center;
border: 1px solid black;
border-top-left-radius: 25px;
border-top-right-radius: 25px;
}
.containerEnd
{
display: flex;
flex-direction: column;
}
.topEnd
{
height: auto;
flex-grow: 1;
}
This Javascript code is used to animate the div elements:
function slideUpFooter() {
var currentHeight = $('#divTop').height();
var footerHeight = $('#divFooter').outerHeight(true);
$('#divTop').animate(
{ height: currentHeight - footerHeight },
2000,
'easeOutBounce',
function () {
$('#divContainer').addClass('containerEnd');
$('#divTop').addClass('topEnd');
});
};
The function called at the end of the animation sets the flexbox parameters, to ensure that the footer sticks to the bottom of the page.

I have updated your Fiddle. Look below for details.
You don't need to use position: fixed for the .foot section, you could use position: relative instead. Since I noticed you were using flex, I took the liberty to fix this using the same.
Changes made
Firstly I suggest adding a div container, giving a class name say - container.
Make the container display: flex & change the default direction to flex-direction: column.
Now since you want the main-content to be scroll-able depending on its contents, you need to first set a height to this section with height: 200px; and then make it scroll-able using overflow-y: auto;
Let me know if you have any doubts.

How can I make my tree view elements overflow past the parent div's width?

I am using bootstrap-treeview to try to make a nice treeview within my MVC project. This control is available on NuGet so its easy to get started with it. The left hand div shows the tree and the right hand div shows the content of each element when clicked:
<body>
<div id="tree" style="position:absolute; width: 20%; height: 100%; overflow: scroll"></div>
<div id="content" class="list-group-item node-tree" style="position: absolute; left: 20%; width: 80%; height: 100%">This is where content goes once you click on a file or folder element.</div>
</body>
There is a slight problem, though. The content of the div with the ID = tree gets cut off:
Ideally, I would like these list elements to overflow to the right, beyond the size of the div with the ID = tree, as you can tell, because I have set overflow: scroll, so I do not want any text to wrap to a new line.
On runtime, it appends list elements as follows...
These list elements seem to have the following CSS:
.list-group-item {
position: relative;
display: block;
padding: 10px 15px;
margin-bottom: -1px;
background-color: #fff;
border: 1px solid #ddd;
}
I have tried adding white-space: nowrap; to this CSS, which makes the text do what I want (and overflow with a scrollbar), but the background and border of each list element stay at the width of the parent (which is not what I want; I want them to also overflow all scrollable width just like I did with the text)!
What can I do to make each element of this list properly overflow past the bounds of the parent div they all exist under?
Edit: I've tried putting overflow:visible on all parent levels as well, but it did not work. It removed the vertical scroll bar and kept all list item background borders still restricted to the width of the tree div. I also found that setting width = 10000px on the .list-group-item CSS partially gives me what I want as well, but obviously this makes the backgrounds too wide and the scroll bar becomes too elongated. I want the width of all list elements to be equal to the width of the widest overflowing content.

I figured it out. I had to change the display to table-row-group and I had to add white-space: nowrap:
.list-group-item {
position: relative;
display: table-row-group;
padding: 10px 15px;
margin-bottom: -1px;
background-color: #fff;
border: 1px solid #ddd;
white-space: nowrap;
}

Add overflow:visible to the parent element(s). You may need this at multiple levels, as each parent element could potentially restrict the content.

How to reveal element by scrolling?

I'm trying to make an effect similar as used on http://www.t-mobile.com/ , when the user scrolls down to the bottom of the page they reveal the "footer" more and more as the user keeps on scrolling.
I've tried to search both here and on google but haven't been able to find anything that's really useful. Most examples only shows/hide the footer once the user scrolls to the bottom.
So my question is, what's the effect called to reveal an element by scrolling? Are there any good tutorials / blog posts about this? All help I can get is much appreciated!

As I commented, you need to make your element fixed, so as explanation goes, I have two elements here, one is a normal position: relative; element, so nothing fancy about that, I assigned relative so that I can make the z-index work
Second element is positioned fixed and also, make sure you use margin-bottom which should be equal to the height of your footer, no need to assign any negative z-index whatsoever to this element.
Demo
Not much HTML ...
<div></div>
<div>Reveal Me</div>
CSS
/* These are for your main site wrapper */
div:first-child {
height: 800px; /* Even auto is fine, I
used fixed height because I don't have any content here */
background: #eee;
margin-bottom: 200px; /* Equals footer wrappers height */
z-index: 1;
position: relative;
}
/* These are for footer wrapper */
div:last-child {
background: #aaa;
height: 200px;
position: fixed;
bottom: 0;
width: 100%;
}
For Dynamic Sizes
Note that am using a fixed height for the fixed positioned element, if you have variable height in footer element, than you need to use JS or jQuery to calculate the height using
$('#wrapperElement').css('margin-bottom', $('#footer').height());
Here, the selectors of #wrapperElement and #footer are my assumed ones, you can replace those with the your own selectors.
Something about fixed element - Horizontal Centering (I think it will be helpful to some users)
When you will make your element fixed, it will get out of the document flow, so if you are assigning fixed to the wrapper of footer element and want to center some content in there, than nest another element inside that wrapper and use width and margin: auto; for that...
Demo 2
HTML
<div></div>
<div>
<div>Reveal Me</div>
</div>
CSS
body > div:first-child {
height: 800px;
background: #eee;
margin-bottom: 200px;
z-index: 1;
position: relative;
}
body > div:last-child {
background: #aaa;
height: 200px;
position: fixed;
bottom: 0;
width: 100%;
}
body > div:last-child div {
width: 80%;
margin: auto;
outline: 1px solid red; /* To show that element is horizontally centered */
}
Note: Selectors used in this answer are too general and are good for
quick demonstration purposes, in real projects, make sure you use
specific selectors

How do I expand selector width from center (or other efficient alternatives)?

I've got a navigational bar (#nav_bar), which has the following CSS properties:
#nav_bar {
background: url(../images/navbar.png) repeat-x;
width: 100%; height: 50px;
margin-bottom: 20px;
}
It takes the width of #wrap which is 1024px wide and has margin: auto;, however I would like to expand it so that it will fit all screen sizes 100%. I attempted to set width: 500%; just to see what it would do, then I realized that it expands from the left -> right, rather than both ways from the center.
So, what I'm asking is;
Is it possible to have an element expand from the center, then
perhaps I could set the max-width property or use javascript to
find out the visitors screen resolution then assign the width from
there; without major inefficiencies, i.e. extended load times/cross-browser compatibility issues?
Just for reference, a link to the particular page I'm talking about
Any answers will be greatly appreciated ;)!

Simply move your #nav_bar out of the #wrap.
Alternatively you can make your #nav_bar have position: absolute; left: 0px; width: 100%; in CSS, that will work too.

Why don't you use CSS3 Media Queries, to find out about screen size of your clients.
If your #nav-bar is a block-level element, like a div, a ul or a p element, then it by default would take the whole width of its container. Thus you don't need to set width: 100%; there. Also, you can use text-align: center; to center align the content.
In your case, you can use absolute positioning with overflow: visible attribute, and set the width of the menu. Also, you may simply extract your #nav-bar out of the wrap, to let it take the whole space.

use margin: auto
you can see an example here: http://jsfiddle.net/s995c/4/