<div class="wrapper">
  <div class="chat-box" *ngIf="visible">
    <div class="chat-box-header">
      <div class="">
        <div class="operator-status">
          {{operator.status}}
          <span class="operator-status-online">●</span>
          <button class="chat-button-header" (click)="toggleChat()">✕</button>
        </div>
        <img [attr.src]="operator.avatar" class="avatar" />
        <h3 class="operator-name">
          {{operator.name}}
        </h3>
      </div>
    </div>
    <div class="chat-box-main">
      <div class="chat-message-bottom" #bottom></div>
      <ng-container *ngFor="let message of messages">
        <div class="chat-message" [class.chat-message-received]="message.type === 'received'" [class.chat-message-sent]="message.type === 'sent'">
          <div class="chat-message-row">
            <div class="chat-message-from-avatar">
              <img [attr.src]="message.from.avatar" class="avatar" />
            </div>
            <div class="chat-message-text">
              {{message.text}}
            </div>
          </div>
          <div class="chat-message-date">
            {{message.date | date: 'short'}}
          </div>
        </div>
      </ng-container>
    </div>
    <div class="chat-box-footer">
      <textarea type="text" class="chat-input-text" placeholder="请输入..." #message (keydown.enter)="onSubmit()" (keyup.enter)="message.value = ''"
        (keyup.escape)="dismiss.emit()"></textarea>
      <button type="submit" class="chat-input-submit" (click)="onSubmit()">↩︎</button>
    </div>
  </div>
  <button mat-fab class="chat-button" (click)="toggleChat()">
    <span *ngIf="visible">✕</span>
    <span *ngIf="!visible">?</span>
  </button>
</div>

./chat-widget.component.scss

.wrapper {
  position: absolute;
  top: 0;
  bottom: 0;
  width: 100vw;
}

.chat-button {
  z-index: 1;
  position: absolute;
  bottom: 20px;
  right: 40px;
  font-size: 32px;
}

.chat-button-header {
  z-index: 1;
  font-weight: bold;
  color: #fff;
  border: 1px solid #fff;
  background-color: inherit;
  padding: 5px 9px;
  margin-left: 5px;
}

.chat-box {
  z-index: 2;
  position: absolute;
  left: 0;
  height: 100vh;
  width: 100vw;
  margin: 0;
  display: flex;
  flex-direction: column;
  box-shadow: 0 5px 40px rgba(0, 0, 0, 0.16);
}

.chat-box-hidden {
  display: none;
}

.chat-box-header {
  padding: 10px;
  color: #fff;
}

.chat-box-main {
  flex: 1;
  width: 100%;
  background: #f5f5f5;
  display: flex;
  flex-direction: column-reverse;
  overflow: auto;
}

.chat-box-footer {
  color: #fff;
  height: 50px;
}

.operator-name {
  margin: 0;
  padding: 1px;
}

.operator-status {
  float: right;
  padding: 4px;
}

.operator-status span {
  margin-left: 4px;
}

.operator-status-online {
  color: lawngreen;
}

.operator-status-offline {
  color: red;
}

.operator-avatar {
  height: 30px;
  width: 30px;
  border-radius: 50%;
  float: left;
  margin-right: 10px;
}

.chat-message {
  display: block;
  width: auto;
  margin: 5px;
  align-self: flex-start;
  flex-direction: row;
  max-width: 80%;
  word-wrap: break-word;
}

.chat-message-date {
  font-size: 11px;
  color: #8d898d;
  padding: 5px;
}

.chat-message-row {
  position: relative;
}

.chat-message-from-avatar {
  height: 35px;
  width: 35px;
  border-radius: 50%;
}

.chat-message-text {
  margin-left: 10px;
  padding: 10px;
  border-radius: 3px;
  position: relative;
}

.chat-message-received {
  margin-right: 50px;
}

.chat-message-received .chat-message-text {
  background: #b9d6f2;
  margin-left: 50px;
  border-bottom-left-radius: 0;
}

.chat-message-received .chat-message-text:before {
  position: absolute;
  content: ' ';
  left: -10px;
  bottom: 0;
  border-right: solid 10px #b9d6f2;
  border-top: solid 10px transparent;
  z-index: 0;
}

.chat-message-received .chat-message-date {
  margin-left: 50px;
}

.chat-message-received .chat-message-from-avatar {
  position: absolute;
  left: 0;
  bottom: -15px;
}

.chat-message-sent {
  align-self: flex-end;
}

.chat-message-sent .chat-message-from {
  float: right;
}

.chat-message-sent .chat-message-text {
  background: #84dccf;
  margin-right: 50px;
  border-bottom-right-radius: 0;
}

.chat-message-sent .chat-message-text:after {
  position: absolute;
  content: ' ';
  right: -10px;
  bottom: 0;
  border-left: solid 10px #84dccf;
  border-top: solid 10px transparent;
  z-index: 0;
}

.chat-message-sent .chat-message-date {
  text-align: right;
  padding-right: 50px;
}

.chat-message-sent .chat-message-from-avatar {
  position: absolute;
  right: 0;
  bottom: -15px;
}

.blue .chat-button {
  background: #1976d2;
}

.blue .chat-box {
  background: #1976d2;
}

.grey .chat-button {
  background: #454549;
}

.grey .chat-box {
  background: #454549;
}

.red .chat-button {
  background: #dd0031;
}

.red .chat-box {
  background: #dd0031;
}

@media (min-width: 576px) {
  .wrapper {
    top: auto;
    right: 0;
    width: 300px;
  }
  .chat-button {
    display: block;
  }
  .chat-button-header {
    display: none;
  }
  .chat-box {
    top: auto;
    left: auto;
    bottom: 100px;
    right: 40px;
    height: 60vh;
    width: 300px;
    border-radius: 10px;
  }
}

@media (min-width: 768px) {
  .chat-box {
    height: 70vh;
  }
}

@media (min-width: 992px) {
  .chat-box {
    height: 80vh;
  }
}

@media (min-width: 1200px) {
  .chat-box {
    /*height: 80vh;*/
  }
}

.avatar {
  height: 30px;
  width: 30px;
  border-radius: 50%;
  float: left;
  margin-right: 10px;
}

.chat-input-text {
  margin: 14px 0 0 14px;
  height: 25px;
  width: 70%;
  border: 0;
  resize: none;
  border: none;
  overflow: auto;
  outline: none;
  box-shadow: none;
  font-size: 14px;
  background-color: inherit;
  color: inherit;
}

.chat-input-text::-webkit-input-placeholder {
  color: inherit;
}

.chat-input-text::-moz-placeholder {
  color: inherit;
}

.chat-input-text::-ms-input-placeholder {
  color: inherit;
}

.chat-input-submit {
  margin: 14px 12px;
  float: right;
  background-color: inherit;
  color: inherit;
  font-size: 24px;
  border: 0;
  outline: none;
}