title: How to apply rounded borders to expanded ExpansionTile in Flutter?
body: <p>What I am trying to do is to apply rounded edges to the entire tile even when the Container inside children is open, in the same way as when it is collapsed.
  I tried to apply the style through its Container using BoxDecoration, but it gives me error. I don't know how to proceed because ExpansionTile unlike ListTile doesn't have an attribute for the shape.</p>
  <p><img src="https://i.stack.imgur.com/Fhr1m.jpg" alt="Collapsed tile with rounded bordes" /></p>
  <p><img src="https://i.stack.imgur.com/kbDUa.jpg" alt="Expanded tile with right angles borders" /></p>
  <pre><code>class DocumentTile extends StatelessWidget {
    final Document document;
  
    const DocumentTile({Key key, this.document}) : super(key: key);
    @override
    Widget build(BuildContext context) {
      return Card(
        margin: const EdgeInsets.only(top: 12, right: 30),
        shape: RoundedRectangleBorder(
          borderRadius: BorderRadius.circular(8.0),
        ),
        color: AppColors.lbBlue.materialColor,
        child: Container(
          width: MediaQuery.of(context).size.width * 0.83,
          child: ExpansionTile(
            tilePadding: const EdgeInsets.only(left: 40.0, right: 30.0),
            backgroundColor: AppColors.nsIconGrey.materialColor,
            trailing: Container(
              width: MediaQuery.of(context).size.width * 0.49,
              child: Row(
                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: [
                  Row(
                    children: [
                      Container(
                        width: 0.5,
                        height: 50,
                        color: Colors.white,
                        margin: const EdgeInsets.only(right: 16.0),
                      ),
                      Text(
                          '${DateTime.fromMicrosecondsSinceEpoch(document.creationDate * 1000)}',
                          style: tileDate),
                    ],
                  ),
                  Row(
                    children: [
                      Container(
                          margin: const EdgeInsets.only(right: 6),
                          height: 35,
                          width: 35,
                          decoration: BoxDecoration(
                            color: AppColors.tagIcon.materialColor,
                            borderRadius: BorderRadius.circular(4),
                          ),
                          child: Center(
                            child: Text(
                              '${document.tags[0].acronym}',
                              style: documentTag,
                            ),
                          )),
                      Container(
                          height: 35,
                          width: 35,
                          decoration: BoxDecoration(
                            color: AppColors.addTagIcon.materialColor,
                            borderRadius: BorderRadius.circular(4),
                          ),
                          child: IconButton(
                            icon: Icon(Icons.add, color: Colors.white),
                            iconSize: 25,
                            padding: const EdgeInsets.all(5.5),
                            onPressed: () {},
                          )),
                    ],
                  ),
                  Row(
                    children: [
                      Container(
                          margin: const EdgeInsets.only(right: 6),
                          height: 35,
                          width: 35,
                          decoration: BoxDecoration(
                            color: AppColors.sPdIcon.materialColor,
                            borderRadius: BorderRadius.circular(4),
                          ),
                          child: IconButton(
                            icon: Icon(Icons.more_vert, color: Colors.white),
                            iconSize: 25,
                            padding: const EdgeInsets.all(5.5),
                            onPressed: () {},
                          )),
                      Container(
                          margin: const EdgeInsets.only(right: 6),
                          height: 35,
                          width: 35,
                          decoration: BoxDecoration(
                            color: AppColors.sPdIcon.materialColor,
                            borderRadius: BorderRadius.circular(4),
                          ),
                          child: IconButton(
                            icon: Icon(Icons.share_outlined, color: Colors.white),
                            iconSize: 20,
                            padding: const EdgeInsets.all(5.5),
                            onPressed: () {},
                          )),
                      Container(
                          height: 35,
                          width: 35,
                          decoration: BoxDecoration(
                            color: AppColors.sPdIcon.materialColor,
                            borderRadius: BorderRadius.circular(4),
                          ),
                          child: IconButton(
                            icon: Icon(Icons.arrow_forward, color: Colors.white),
                            iconSize: 25,
                            padding: const EdgeInsets.all(5.5),
                            onPressed: () {
                              Navigator.pushNamed(context, '/documentDetail',
                                  arguments: document.id);
                            },
                          )),
                    ],
                  ),
                ],
              ),
            ),
            subtitle: Text(
              '${document.abstract0}',
              style: tileDescription,
              overflow: TextOverflow.ellipsis,
              maxLines: 1,
            ),
            title: Text(
              '${document.title}',
              style: tileTitle,
              overflow: TextOverflow.ellipsis,
              maxLines: 1,
            ),
            children: [
              Container(
                width: double.maxFinite,
                padding: const EdgeInsets.only(
                    left: 40.0, right: 30.0, top: 20, bottom: 20),
                color: Color(0xFF2A3141),
                child: Text(
                  '${document.content}',
                  style: TextStyle(
                      color: AppColors.darkerText2.materialColor,
                      fontSize: 10,
                      fontWeight: FontWeight.w300),
                  maxLines: 3,
                  overflow: TextOverflow.ellipsis,
                ),
              )
            ],
          ),
        ),
      );
    }
  }
  
  </code></pre>