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To get some part of html page to be non-scrollable and invisible

there is a html page of 400vh height and divided into two parts. I want the first part to be hidden and importantly non-scrollable by having a button
for that, the way once the user clicks the button, second part supersedes the first and the whole hight turns to 200vh so that only the second part is visible and scrollable.
how to do so?

Requirement is not very clear. But I guess you would hide the first div with css
diplay: none
then you would have the second div set as:
overflow-y: auto
And if this has to be done with a button, on click, link a js method which would update the style attributes to the desired css elements.

Creating focus on element that's behind a different element

all. I'm a beginner trying to create my own website.
For an input field, I want the text to appear behind horizontal lines. To achieve this, I created a container div element with many sub divs each two pixels high with a bottom border of one pixel, then placed the container div in front of the input field.
The problem is that when hovering over the input field, the cursor only changes into the "text" style cursor in the small area at the very left edge of the input field not covered by the horizontal lines created by the sub div elements.
What I would like to happen, and I fear I may need Javascript, is to have the cursor, when hovering over the container div's horizontal lines, turn into the "text" cursor and when clicked for the focus to activate within the input field.
I hope my question isn't confusing. Thank you in advance.

Add this CSS to your div with horizontal lines:
pointer-events: none;
This indicates that your div element should not become the target of mouse events, effectively letting them "pass through" it.
For more information, see the MDN article on pointer-events.

Assuming that I'm understanding your question, you do need to use JavaScript. Using jQuery you can do the following:
$('.horizontalLines').click(function() {
$('.inputField').focus();
});
This just makes it so that clicking one div will select the text input box.

Hide part view text in overflow hidden

I making bookReader and one page will fit to user viewport. For this approach i make div
<div id="book">Long text...</div>
with
#book {
overflow: hidden
}
Then i provide to scroll this element only with buttons next and prev. When user click next button it will scroll like this
$('#next').on('click',function(){
$('#book').scrollTop($('#book').scrollTop() + $('#book').height());
});
But i have problem. That some times user will see part of text-line. How can i check every page is that last text-line is shown broken, and hide them if him is broken. I don't want to change content. I need a function that will check every page if that contains that part text and if this have that part text, hide him by making on the top element that should hide that.
DEMO: I show in red color what i need find and hide.
http://jsfiddle.net/lvivgeorge/jd7mum6c/3/
DEMO 2: It can contains anything (header tags, img, <code> , etc.)
http://jsfiddle.net/lvivgeorge/jd7mum6c/12/
P.S. Make all the same line-height is not solution. Change content styles is not solution too.

IMHO You should set fixed line-height of each text line, and set container height fixed as a multiple height of each textline. This should helps
Check this out: http://jsfiddle.net/jd7mum6c/5/

Add Hover display for table cell

I have a table, but it is not in a list format, meaning not a column/row format. I look for specific cells to add a hover event that displays a description of the cell.
$("#TableID td").hover(function(){
//ifCellThatIWant
$(this).append("<span>Message that was brought in</span>");
},
function(){
$(this).children().remove();
});
The problem is right now is that the hover displays a span(with info. inside) that I used jquery to append the span to the cell when mouseover, which expands the cell, which is an effect that I don't like or want. I'm Trying to have an out of the table look,but still be by the cell that triggered the event; because if the span has a lot of info. in it, expanding the cell dynamically will start to look pretty nasty. Also will help if I had some type of css direction on how will I make the display for the mouseover "description" span look nice. My mind thought is that a way to accomplish what I want is giver the appended span the position of my mouse cursor when hover, but not sure if its the right approach or what the syntax would look like.

Make the span display as block and set the z-index greater than anything else on the page. Then you can absolute position it and set the left and top properties to the x and y positions of the mouse location.
EDIT:
Here's a demo of what I mean --> http://jsbin.com/odape. Instead of appending a span, I would suggest just creating a placeholder one at the bottom of your html to use for each cell and just change the text to display (not sure how you were bringing it in so I didn't add it in my example.

How to show/hide html controls keeping their locations the same?

Imagine search bar on html page, it has, say 4 controls on the same line, each wrapped in DIV.
E.g. a few listboxes on the same line:
searchbyX, byY, byZ, byN
After some clicks, some of these controls are hidden, some are displayed (using JQuery).
The problem is that I want each control to keep it's place, but if I hide it's left neighbour, it will move to the left, thus not keeping its original position.
How to show/hide controls keeping their locations the same?

If I understand you correctly,
visibility: hidden
will do the trick.
It will hide the element, but reserve the space it needs.
To make it visible again, remove the property or set a explicit
visibility: visible

Instead of hiding them by setting "display" to "none", set "visibility" to "hidden".