title: Bootstrap 3 - Modal backdrop doesn't resize according to the height of modal dialog?
body: <p>I put a form inside a modal and I trying to show some of the hidden fields of the form when users trigger the radio-button. After I show the hidden fields the height of the modal auto-rescale but the height of the modal's backdrop doesn't work as the modal dialog. How could I fix it?</p>
  
  <pre><code>&lt;!-- modal --&gt;
  &lt;div class='modal fade' id='contact-modal' tabindex='-1' role='dialog' aria-labelledby='modal-label' aria-hidden='true' data-backdrop='static' data-keyboard='false'&gt;
      &lt;div class='modal-dialog'&gt;
          &lt;div class='modal-content'&gt;
  
              &lt;div class='modal-header'&gt;
                  &lt;button type='button' class='close' data-dismiss='modal'&gt;
                      &lt;span aria-hidden='true'&gt;&amp;times;&lt;/span&gt;&lt;span class='sr-only'&gt;Close&lt;/span&gt;
                  &lt;/button&gt;
                  &lt;h4 class='modal-title' id='modal-label'&gt;今日聯絡事項&lt;/h4&gt;
              &lt;/div&gt;
  
              &lt;div class='modal-body'&gt;
                  &lt;form role='form'&gt;
                      &lt;div class='form-group'&gt;
                          &lt;label for='learning'&gt;學習情況&lt;/label&gt;
                          &lt;div class='btn-group' id='learning' data-toggle='buttons'&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;良好&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;尚可&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;不佳&lt;/label&gt;
                          &lt;/div&gt;
                      &lt;/div&gt;
                      &lt;div class='form-group'&gt;
                          &lt;label for='lunch'&gt;用餐情形&lt;/label&gt;
                          &lt;div class='btn-group' id='lunch' data-toggle='buttons'&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;食慾佳&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;食量正常&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;食慾不佳&lt;/label&gt;
                          &lt;/div&gt;
                      &lt;/div&gt; 
                      &lt;div class='form-group'&gt;
                          &lt;label for='sleep'&gt;午睡情況&lt;/label&gt;
                          &lt;div class='btn-group' id='sleep' data-toggle='buttons'&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;良好&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;晚睡&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' /&gt;不睡&lt;/label&gt;
                          &lt;/div&gt;
                      &lt;/div&gt;  
                      &lt;div class='form-group'&gt;
                          &lt;label for='eat-medicine'&gt;餵藥紀錄&lt;/label&gt;
                          &lt;div class='btn-group'  id='eat-medicine' data-toggle='buttons'&gt;
                              &lt;label class='btn btn-default active'&gt;&lt;input type='radio' value='0' /&gt;無&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' value='1' /&gt;有&lt;/label&gt;
                          &lt;/div&gt;
                          &lt;div class='hide' id='take-time'&gt;
                              &lt;input type='text' class='form-control' placeholder='第一次餵藥時間' /&gt;
                              &lt;input type='text' class='form-control' placeholder='第二次餵藥時間' /&gt;
                              &lt;input type='text' class='form-control' placeholder='第三次餵藥時間' /&gt;
                          &lt;/div&gt;
                      &lt;/div&gt; 
                      &lt;div class='form-group'&gt;
                          &lt;label for='health'&gt;身體狀況&lt;/label&gt;
                          &lt;div class='btn-group' id='health' data-toggle='buttons'&gt;
                              &lt;label class='btn btn-default active'&gt;&lt;input type='radio' value='0' /&gt;正常&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' value='1' /&gt;異常&lt;/label&gt;
                          &lt;/div&gt;&lt;br /&gt;
                          &lt;div class='btn-group hide' id='symptom' data-toggle='buttons'&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' value='cough' /&gt;咳嗽&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' value='runny-nose' /&gt;流鼻涕&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' value='sneeze' /&gt;打噴嚏&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' value='fever' /&gt;發燒&lt;/label&gt;
                              &lt;label class='btn btn-default'&gt;&lt;input type='radio' value='other' /&gt;其他&lt;/label&gt;
                          &lt;/div&gt;
                          &lt;div class='hide' id='cause'&gt;
                              &lt;input type='text' class='form-control' placeholder='原因' /&gt;
                          &lt;/div&gt;
                      &lt;/div&gt;&lt;br /&gt;
                      &lt;div class="form-group"&gt;
                          &lt;label for='teacher-say'&gt;老師說&lt;/label&gt;
                          &lt;textarea class="form-control" rows="7" id="teacher-say" placeholder="訊息內容..."&gt;&lt;/textarea&gt;
                      &lt;/div&gt;
                  &lt;/form&gt;
              &lt;/div&gt;
  
              &lt;div class='modal-footer'&gt;
                  &lt;button type='button' class='btn btn-default' data-dismiss='modal'&gt;取消&lt;/button&gt;
                  &lt;button type='button' class='btn btn-primary' data-dismiss='modal'&gt;確定&lt;/button&gt;
              &lt;/div&gt;
  
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>