Joining diagonal divs with flex - html

Question

I have 2 divs. Right div is an image cutted diagonally. Left divs must have some text inside. I want this to be fully responsive like this:
The problem occurs when I change window size, it's collapsing like in the image:
.
Also there is a text on left div that need to be displayed, but with flex this seems not to work so i disabled it. Please provide solution for this.
Here is my css and html:
#diagonal {
display: flex;
width: 100%;
}
#diagonal #ct-about-col-left {
width: 60%;
border-right: 190px solid transparent;
border-bottom: 500px solid grey;
z-index: 2;
}
#diagonal span {
display: none;
}
#ct-about-col-right {
height: 500px;
width: 50%;
border: 2px solid;
background-image: url(images/content/about/right-col-image.jpg);
z-index: 0;
margin-left: -12%;
margin-right: 0;
}
}
<div id="diagonal">
<div id="ct-about-col-left">
<span>We are the best</span>
<span>text1 text1 text1</span>
<span>Text2 text2 text2 text2</span>
<div>
<span>Read more</span>
</div>
</div>
<div id="ct-about-col-right"></div>
</div>

Answer 1

Maybe consider a slightly different mark-up and method of adding the picture (as a background-image) and making the angle (with transform: skew).
Live Demo: http://codepen.io/anon/pen/rjyKRo
<div class="container">
<div class="caption">
<p>CONTENT</p>
</div>
</div>
* { box-sizing: border-box; }
.container {
width: 100%;
height: 50vh;
overflow: hidden;
background-image: url("http://unsplash.it/600");
background-size: cover;
background-repeat: no-repeat;
background-position: 100% 50%;
}
.caption {
height: 100%;
width: 50%;
min-width: 500px;
padding-top: 20%;
padding-left: 130px;
background-color: #ddd;
transform: skew(10deg, 0deg);
transform-origin: 100% 100%;
}
.caption p {
transform: skew(-10deg, 0deg);
}

Answer 2

May I suggest another approach which will save You some markup space and CSS rules as well.
Simply create a full-width div with the ID of lets say ct-about, give it a background color grey and then simply chain the image background on top of the color like so:
background: url('images/content/about/right-col-image.jpg') no-repeat right top, grey;
This simply tells the browser, make my box grey and put that image over the grey color. The no-repeat right top properties are preventing the browser from repeating the image so you don't get a tile, tell ti to place the image on the far right and top positions.
This way everything will be responsive as well.
Here is a Fiddle for You to better understand.
You can find more information about multiple CSS backgrounds in the Mozilla Developer Network