.htCommentCell {
    position: relative;
}

.htCommentCell:after {
    content: '';
    position: absolute;
    top: 0;
    right: 0;
    border-left: 6px solid transparent;
    border-top: 6px solid black;
}

.htComments {
    display: none;
    z-index: 1059;
    position: absolute;
}

.htCommentTextArea {
    box-shadow: rgba(0, 0, 0, 0.117647) 0 1px 3px, rgba(0, 0, 0, 0.239216) 0 1px 2px;
    border: none;
    border-left: 3px solid #ccc;
    background-color: #fff;
    width: 215px;
    height: 90px;
    font-size: 12px;
    padding: 5px;
    outline: 0px !important;
    -webkit-appearance: none;
}

.htCommentTextArea:focus {
    box-shadow: rgba(0, 0, 0, 0.117647) 0 1px 3px, rgba(0, 0, 0, 0.239216) 0 1px 2px, inset 0 0 0 1px #5292f7;
    border-left: 3px solid #5292f7;
}

.htCommentCell {
    position: relative;
}

.htCommentCell:after {
    content: '';
    position: absolute;
    top: 0;
    right: 0;
    border-left: 6px solid transparent;
    border-top: 6px solid black;
}

.htComments {
    display: none;
    z-index: 1059;
    position: absolute;
}

.htCommentTextArea {
    box-shadow: rgba(0, 0, 0, 0.117647) 0 1px 3px, rgba(0, 0, 0, 0.239216) 0 1px 2px;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
    border: none;
    border-left: 3px solid #ccc;
    background-color: #fff;
    width: 215px;
    height: 90px;
    font-size: 12px;
    padding: 5px;
    outline: 0px !important;
    -webkit-appearance: none;
}

.htCommentTextArea:focus {
    box-shadow: rgba(0, 0, 0, 0.117647) 0 1px 3px, rgba(0, 0, 0, 0.239216) 0 1px 2px, inset 0 0 0 1px #5292f7;
    border-left: 3px solid #5292f7;
}

Add box-sizing:border-box to the Comments editor.
Add box-sizing:border-box to the Comments editor.
