Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
    <script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
    <style>
        textarea {
            width:100%;
            height:100%;
        }
        .modal-custom {
            width: 70%;
            height: 70%;
            padding: 0;
        }
        .modal-content-custom {
            height: auto;
            min-height: 100%;
            border-radius: 0;
        }
    </style>
</head>
<body>
<button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#editForm">
    Launch demo modal
</button>


<div class="modal fade editForm" id="editForm" tabindex="-1" role="dialog" aria-labelledby="editFormLabel">
    <div class="modal-dialog modal-custom" role="document">
        <div class="modal-content modal-content-custom">
            <div class="modal-header">
                <h4 class="modal-title" style="display: inline-block" id="editFormLabel">Source code</h4>
            </div>
            <div class="modal-body">
                <textarea id="textpage" class="edtextarea" title="Source code of page"></textarea>
            </div>
            <div class="modal-footer">
                <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
            </div>
        </div>
    </div>
</div>


</body>
</html>