absolute positioning inside a table

问题

I need to position something with absolute positioning inside a td. To get around the fact that a td is undefined when setting it to relative, I use a div set to relative inside my td then inside that div I have an inner div set to absolute. This all works great when I have content filling up the td. When I set the content of the td to display none then the absolute positioning gets all screwed up.

Does anyone know why this would be.

HTML:

<table>
    <tr>
        <td>
            <div class="relative">
                <div class='absolute'>
                    <p>A</p>
                </div>
            </div>
            <div class="slot"></div>
            <div class="slot"></div>
        </td>
        <td>
          <div class="relative">
             <div class='absolute'>
               <p>B</p>
           </div>
           </div>
           <div class="slot hidden"></div>
           <div class="slot"></div>
        </td>
    </tr>
</table>

And CSS:

td{
    border:1px solid red;
    width:100px;
    height:60px;
    vertical-align:bottom;
}

.slot{
  width:100px;
  height:29px;
  background-color:#999;
  border:1px dashed blue;
}

.relative{
    position:relative;
}

.absolute{
    position:absolute;
    top:5px;
    left:5px;
}
.hidden{
    display:none;
}

And a live version: http://jsfiddle.net/HgEtQ/

In the fiddle above you can see the first table cell works correctly. The absolutely positioned div is in the correct space. The second one has hidden the top slot and now the absolute positioning is not in the top left corner anymore. If you take out both slots then it really screws up the absolute positioning. I need to positioning it absolute because some of the elements will be shifted depending on the element.

回答1:

There are a couple things going on here.

You have this:

td {
    /*...*/
    vertical-align:bottom;
}

That will push the content of the cells to the bottom.

Also, an absolutely positioned element is removed from the normal flow so it won't contribute to its parent's height:

It is removed from the normal flow entirely (it has no impact on later siblings). An absolutely positioned box establishes a new containing block for normal flow children and absolutely (but not fixed) positioned descendants.

In particular, this means that your div.relative elements have a height of zero but they will still have an upper left corner so your absolute positioning offsets are anchored somewhere.

Then you have <div class="slot hidden"> and the hidden removes the <div> from the layout so you effectively have just this:

<div class="relative"></div> <!-- Height zero -->
<div class="slot"></div>     <!-- Height 29px -->

That combined with the vertical-align: bottom means that your second div.absolute will be positioned 5px from the top of the div.slot and that is 25px from the bottom of the table cell.

The first cell works fine because the two visible div.slot elements push the div.relative right to the top of the cell, then the div.absolute is positioned 5px from the top of the div.relative and that is 5px from the top of the table cell.

Unfortunately, adding position: relative to a <td> is a bit dodgy as far as browsers go so you'll need some hackery to get your positioning right while keeping vertical-align: bottom. You could re-structure the <td>s like this:

<td>
    <div class="relative">
        <div class='absolute'>
            <p>A</p>
        </div>
    </div>
    <div class="nonsense">
        <div class="slot"></div>
        <div class="slot"></div>
    </div>
</td>

And the CSS like this:

td{
    border:1px solid red;
    width:100px;
    height:60px;
    vertical-align: top;
}

.slot{
    width:100px;
    height:29px;
    background-color:#999;
    border:1px dashed blue;
}

.relative {
    position:relative;
}
.nonsense {
    height: 62px; /* td[height] + 2 for the borders */
    display: table-cell;
    vertical-align: bottom;
}

.absolute{
    position:absolute;
    top:5px;
    left:5px;
}
.hidden{
    display:none;
}

Live example: http://jsfiddle.net/ambiguous/aV4nT/

Or you could use visibility: hidden:

hidden
The generated box is invisible (fully transparent, nothing is drawn), but still affects layout. Furthermore, descendants of the element will be visible if they have 'visibility: visible'.

instead of display: none for your .hidden class:

.hidden {
    visibility: hidden;
}

This will leave both div.slot elements taking up space and affecting the layout but only the second one will be seen.

Live example: http://jsfiddle.net/ambiguous/RcdNh/

来源：https://stackoverflow.com/questions/6798532/absolute-positioning-inside-a-table

标签

html

css

positioning