unera/gist:f9b4b1041fa1c6e36a91592a6a1fd12a

## gistfile1.html
<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport"
            content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">

        <title>тест</title>

        <style>
            .fill {
                width: 100%;
                height: 100%;
            }

            body > div.section {
                position: fixed;
                left: 0;
                right: 0;
                top: 0;
                bottom: 0;
                overflow: hidden;
            }

            .flex {
                display: flex;
                flex-direction: row;
                flex-wrap: nowrap;
                overflow: hidden;
                padding: 0px;
                margin: 0px;
            }

            .flex >* {
                padding: 0px;
                margin: 0px;
                overflow: hidden;
                flex: 0 0 auto;
            }

            .flex.top {
                flex-direction: column;
            }
            .flex.left {
                flex-direction: row;
            }

            .flex > .size-3     { flex-basis: 12.5%;    }
            .flex > .size-6     { flex-basis: 25%;      }
            .flex > .size-12    { flex-basis: 50%;      }
            .flex > .size-18    { flex-basis: 75%;      }
            .flex > .size-21    { flex-basis: 87.5%;    }
            .flex > .size-24    { flex-basis: 100%;     }

            .overflow-y {
                overflow-y: auto;
                -webkit-overflow-scrolling: touch;
            }

            .red    { border: 1px solid red;            }
            .green  { border: 1px solid green;          }
            .dashed.red {   border: 1px dashed red;     }
            .dashed.green { border: 1px dashed green;   }
            .dotted {       border: 1px dotted black;   }


            *::before, *::after {
                box-sizing: border-box;
            }
        </style>
    </head>
    <body>
        <div class="section flex top">
            <div class="size-3 flex left control-panel red">
                <div class="size-6 green">
                    <button class="fill dotted">A</button>
                </div>

                <div class="size-12 green">
                    <button class="fill dotted">B</button>
                </div>

                <div class="size-6 green">
                    <button class="fill dotted">C</button>
                </div>
            </div>

            <div class="size-21">
                <div class="flex fill top red dashed">
                    <div class="size-18 dashed green">контент</div>
                    <div class="size-6 green">нижний блок</div>
                </div>
            </div>
        </div>
    </body>
</html>
	<!DOCTYPE html>
	<html lang="en">
	<head>
	<meta charset="utf-8">
	<meta http-equiv="X-UA-Compatible" content="IE=edge">
	<meta name="viewport"
	content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">

	<title>тест</title>

	<style>
	.fill {
	width: 100%;
	height: 100%;
	}

	body > div.section {
	position: fixed;
	left: 0;
	right: 0;
	top: 0;
	bottom: 0;
	overflow: hidden;
	}

	.flex {
	display: flex;
	flex-direction: row;
	flex-wrap: nowrap;
	overflow: hidden;
	padding: 0px;
	margin: 0px;
	}

	.flex >* {
	padding: 0px;
	margin: 0px;
	overflow: hidden;
	flex: 0 0 auto;
	}

	.flex.top {
	flex-direction: column;
	}
	.flex.left {
	flex-direction: row;
	}

	.flex > .size-3 { flex-basis: 12.5%; }
	.flex > .size-6 { flex-basis: 25%; }
	.flex > .size-12 { flex-basis: 50%; }
	.flex > .size-18 { flex-basis: 75%; }
	.flex > .size-21 { flex-basis: 87.5%; }
	.flex > .size-24 { flex-basis: 100%; }

	.overflow-y {
	overflow-y: auto;
	-webkit-overflow-scrolling: touch;
	}

	.red { border: 1px solid red; }
	.green { border: 1px solid green; }
	.dashed.red { border: 1px dashed red; }
	.dashed.green { border: 1px dashed green; }
	.dotted { border: 1px dotted black; }


	::before, ::after {
	box-sizing: border-box;
	}
	</style>
	</head>
	<body>
	<div class="section flex top">
	<div class="size-3 flex left control-panel red">
	<div class="size-6 green">
	<button class="fill dotted">A</button>
	</div>

	<div class="size-12 green">
	<button class="fill dotted">B</button>
	</div>

	<div class="size-6 green">
	<button class="fill dotted">C</button>
	</div>
	</div>

	<div class="size-21">
	<div class="flex fill top red dashed">
	<div class="size-18 dashed green">контент</div>
	<div class="size-6 green">нижний блок</div>
	</div>
	</div>
	</div>
	</body>
	</html>