title: bootstrap form-group separating textbox from button
body: <p>I'm trying to make a form-group with a textbox and a button using asp.net and bootstrap.</p>
  
  <p>Code:</p>
  
  <pre><code>&lt;div class="col-sm-4 col-md-4 col-lg-4 input-group"&gt;
      &lt;asp:TextBox ID="txtPN" runat="server" placeholder="Search" CssClass="form-control" ClientIDMode="Static" /&gt;
      &lt;span class="input-group-btn"&gt;
          &lt;asp:Button ID="btnSearchPN" runat="server" Text="Search" CssClass="btn btn-default" /&gt;
      &lt;/span&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>This works just fine:</p>
  
  <p><a href="https://i.stack.imgur.com/UWwAi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UWwAi.png" alt="Small window size"></a></p>
  
  <p>However, if I expand the browser's window, the textbox and the button got separated:</p>
  
  <p><a href="https://i.stack.imgur.com/AHMiJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/AHMiJ.png" alt="enter image description here"></a></p>
  
  <p>This group is inside <code>&lt;div class="container-fluid"/&gt;</code>. The entire code follows:</p>
  
  <pre><code>&lt;div class="container-fluid"&gt;
      &lt;div class="row"&gt;
          &lt;div class="col-sm-4 col-md-4 col-lg-4 form-group"&gt;
              &lt;label for="cbMarcas"&gt;Selecione a marca:&lt;/label&gt;
              &lt;asp:DropDownList ID="cbMarcas" CssClass="form-control" runat="server" ClientIDMode="Static" DataTextField="Descricao" DataValueField="Id" OnSelectedIndexChanged="cbMarcas_SelectedIndexChanged" AutoPostBack="True"&gt;&lt;/asp:DropDownList&gt;
          &lt;/div&gt;
  
          &lt;div class="col-sm-4 col-md-4 col-lg-4 form-group"&gt;
              &lt;label for="cbModelos"&gt;Selecione o modelo:&lt;/label&gt;
              &lt;asp:DropDownList ID="cbModelos" CssClass="form-control" runat="server" ClientIDMode="Static" DataTextField="Descricao" DataValueField="Id"&gt;&lt;/asp:DropDownList&gt;
          &lt;/div&gt;
  
          &lt;div class="col-sm-4 col-md-4 col-lg-4 input-group"&gt;
              &lt;asp:TextBox ID="txtPN" runat="server" placeholder="Search" CssClass="form-control" ClientIDMode="Static" /&gt;
              &lt;span class="input-group-btn"&gt;
                  &lt;asp:Button ID="btnSearchPN" runat="server" Text="Search" CssClass="btn btn-default" /&gt;
              &lt;/span&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>What I'm doing wrong?</p>