添加前后按钮后<td>溢出<table>容器的原因咨询

我在包含asp:DropDownList的<td>元素中添加了上一页（Previous）和下一页（Next）按钮，添加后该<td>元素溢出了<table>的宽度。已排查确认<table>的class="DataEntry"和asp:DropDownList的CssClass="rMidiEntrySelect"并非问题原因，尝试将white-space:nowrap直接设置在<td>上并移除<div>，但无效。

完整表格代码片段 #

<table class="DataEntry" border="1">
  <caption>Adding New Record</caption>
  <tr>
    <th scope="col" colspan="3">&nbsp;&nbsp;Add To Invoice&nbsp;&nbsp;</th>
  </tr>
  <tr>
    <td colspan="3" >
     <div style="white-space:nowrap;">
       <asp:ImageButton ID="PreviousInvoiceAdding" runat="server" ImageUrl="~/Images/Previous16x16.png" OnClick="PreviousInvoice_Click" ToolTip="Previous" />
       <asp:DropDownList ID="DropDownListInvoice" runat="server" AutoPostBack="true" CssClass="rMidiEntrySelect" OnSelectedIndexChanged="DropDownListInvoice_SelectedIndexChanged" />
       <asp:ImageButton ID="NextInvoiceAdding" runat="server" ImageUrl="~/Images/Next16x16.png" OnClick="NextInvoice_Click" ToolTip="Next" />
     </div>
    </td>
  </tr>
  <tr>
    <th scope="col">&nbsp;&nbsp;Cost&nbsp;&nbsp;</th>
    <th scope="col" colspan="2">&nbsp;&nbsp;Date Purchased&nbsp;&nbsp;</th>
  </tr>
  <tr>
    <td>
       <asp:TextBox ID="Cost" runat="server" CssClass="rMidiEntryText" MaxLength="8"/>
    </td>
    <td colspan="2">
      <div style="text-align: center">
         <asp:Label ID="DatePurchased" runat="server" class="rMidiEntryLabel"/>
      </div>
    </td>
  </tr>
  <tr>
    <th scope="col">&nbsp;&nbsp;Volume Type&nbsp;&nbsp;</th>
    <th scope="col">&nbsp;&nbsp;Measurement&nbsp;&nbsp;</th>
    <th scope="col">&nbsp;&nbsp;Units&nbsp;&nbsp;</th>
  </tr>
  <tr>
    <td>
       <select ID="SelectVolumeType" name="Measurement" runat="server" class="rMidiEntrySelect" disabled >
           <option value="0">Liquid</option>
           <option value="1">Weight</option>
       </select>
    </td>
    <td>
       <asp:DropDownList  ID="SelectMeasurement" runat="server" AutoPostBack="true" CssClass="rMidiEntrySelect" />
    </td>
    <td>
        <asp:TextBox ID="Units" runat="server" CssClass="rMidiEntryText" />
    </td>
  </tr>
  <tr>
    <th scope="col" colspan="3">&nbsp;&nbsp;Invoice Reference&nbsp;&nbsp;</th>
  </tr>
  <tr>
    <td colspan="3">
        <asp:TextBox ID="InvoiceRef" runat="server" CssClass="rMidiEntryText" MaxLength="100"/>
    </td>
  </tr>
</table>

渲染效果 #

添加按钮后，包含下拉框和前后按钮的TD元素超出了表格边界。

移除按钮后的代码及效果 #

代码片段 #

<table class="DataEntry" border="1">
  <caption>Adding New Record</caption>
  <tr>
    <th scope="col" colspan="3">&nbsp;&nbsp;Add To Invoice&nbsp;&nbsp;</th>
  </tr>
  <tr>
    <td colspan="3" >
      <asp:DropDownList ID="DropDownListInvoice" runat="server" AutoPostBack="true" CssClass="rMidiEntrySelect" OnSelectedIndexChanged="DropDownListInvoice_SelectedIndexChanged" />
    </td>
  </tr>
  .
  .
  .
</table>

渲染效果 #

移除按钮后，下拉框完全容纳在表格的TD元素内，未出现溢出情况。

相关CSS代码 #

table.DataEntry {
border-color: #00274C;
border-style: solid;
border-width: 2px;
border-collapse: collapse;
padding: 4px; 
color: white;
font-size: 10pt;
}

table.DataEntry caption {
background-color: #004280;
color: #FFD11A;/* #FFCB05;*/
border-color: #00274C;
border-style: solid;
border-width: 2px;
border-collapse: collapse;
padding: 4px;
font-weight: bold;
font-size: 12pt;
white-space: nowrap;
text-decoration: underline;
text-underline-position: under;
text-align:center; 
caption-side: top;
vertical-align: top;
}

table.DataEntry th {
background-color: #00274C;
color: #FFCB05;
text-align:center; 
}

.rMidiEntryLabel {
width: 100%;
color: white;
background-color: black; 
padding-left: 4px;  
padding-right: 2px;
border-style: none;
text-align: center !important;
}

.rMidiEntryText {
width: 100%;
color: white;
background-color: black; 
padding-left: 4px;  
padding-right: 2px;
border-style: none;
text-align: center !important;
}

.rMidiEntrySelect {
width: 100%;
color: white;
background-color: black; 
padding-left: 4px;  
padding-right: 2px;
text-align: center !important;
}

其余CSS为Bootstrap，暂不认为相关。

生成的HTML输出 #

<tr>
  <td colspan="3" >
   <div style="white-space:nowrap;">
     <input type="image" name="ctl00$MainContent$PreviousInvoiceAdding" id="MainContent_PreviousInvoiceAdding" title="Previous" src="Images/Previous16x16.png" />
     <select name="ctl00$MainContent$DropDownListInvoice" onchange="javascript:setTimeout('__doPostBack(&quot;ctl00$MainContent$DropDownListInvoice&quot;,&quot;&quot;)', 0)" id="MainContent_DropDownListInvoice" class="rMidiEntrySelect">
  <option selected="selected" value="ee29b399-34a7-4024-819d-790c9903d4b5~12/11/2023 12:00:00 PM">PayPal: 53S99726XA966511X Brian Garmins Jr.</option>
  <option value="3013ce76-18e3-4458-b4c0-1c17ca46a503~11/28/2023 12:00:00 PM">PayPal: 6F945766E4687834E Nelson Laquindanum</option>
  <option value="722137b5-3201-408a-bd90-9cac6d72829f~11/23/2023 12:00:00 PM">Amazon 111-3584319-8686651</option>
  <option value="4aee9017-8930-4028-ad3f-ff1c494e91bb~10/30/2023 12:00:00 PM">PayPal: 8FL63970TJ5988418 Shake Ums</option>
  <option value="b9995175-f516-4366-9b30-32f03830c724~10/25/2023 12:00:00 PM">PayPal: 4WR12564FW882444X Brett Shumock</option>
  <option value="ec0f668b-1fe7-4424-9264-fc0825b9e999~10/18/2023 12:00:00 PM">PayPal: 6L091961CE7545104 Brandon Reigert</option>
  
  </select>
     <input type="image" name="ctl00$MainContent$NextInvoiceAdding" id="MainContent_NextInvoiceAdding" title="Next" src="Images/Next16x16.png" />
   </div>
  </td>
</tr>

疑问 #

当前结果似乎违背了基础HTML规则：TD是TR的子元素，TR是TABLE的子元素，TABLE作为容器应包含所有内部元素，内部元素不应超出TABLE边界，特此寻求原因解析。

内容的提问来源于stack exchange，提问作者RSullivan