Notes about the layout strategy in mozilla

Bernd Mielke

Specs

The code

The overload of the Initialize and BalanceColumnWidths routines depends on the table style and is decided at runtime. On the first reflow nsTableFrame::BalanceColumnWidths is called. Inside this routine the corresponding table layout strategy is created and initialized (BasicTableLayoutStrategy::Initialize). Later the column widths are balanced by BasicTableLayoutStrategy::BalanceColumnWidths or the fixed layout counter part. In the following the Basic Table Layout Strategy is discussed. The functionality is distributed over a large number of subroutines as shown in the chart below.

The words

Minimum content width - min

the minimum width that is required to layout the content, all linebreak possibilities will be used

percent width - pct

the cell width specified in percent,

fixed width - fix

the cell width specified as px, mm etc.,

proportional width - prop

the cell width specified via 1*, 2* etc. and the

desired width - des

the width the content could fill without any linebreaks.

#define WIDTH_NOT_SET   -1
#define NUM_WIDTHS       10
#define NUM_MAJOR_WIDTHS 3 // MIN, DES, FIX
#define MIN_CON          0 // minimum width required of the content + padding
#define DES_CON          1 // desired width of the content + padding
#define FIX              2 // fixed width either from the content or cell, col, etc. + padding
#define MIN_ADJ          3 // minimum width + padding due to col spans
#define DES_ADJ          4 // desired width + padding due to col spans
#define FIX_ADJ          5 // fixed width + padding due to col spans
#define PCT              6 // percent width of cell or col 
#define PCT_ADJ          7 // percent width of cell or col from percent colspan
#define MIN_PRO          8 // desired width due to proportional <col>s or cols attribute
#define FINAL            9 // width after the table has been balanced, considering all of the others

The priority of allocations for columns is as follows:

1. max(MIN_CON, MIN_ADJ)
2. max (PCT, PCT_ADJ)
3. FIX
4. FIX_ADJ
5. max(DES_CON, DES_ADJ), but use MIN_PRO if present
6. for a fixed width table, the column may get more space if the sum of the col allocations is insufficient

Column width info

Special cases

width = "0*"

<table width="200px" border>
 <tbody>
 <tr><td>foo</td><td width="100%">bar</td></tr>
 </tbody>
</table>

<table border width="200px">
 <col width="0*"><col>
 <tbody>
  <tr><td>foo</td><td>bar</td></tr>
 </tbody>
</table>

Effective columns

<table width="200px" border>
<tbody>
 <tr><td>foo</td><td colspan="2" width="120px">bar</td></tr>
 <tr><td>baz</td><td>zap</td></tr>
</tbody></table>

Overspeficied Percent Columns

<table border width="200px">
 <col width="30%"><col width="60%">
 <tbody>
  <tr><td>foo</td><td>bar</td></tr>
 </tbody>
</table>

<table border width="200px">
 <col width="40%"><col width="80%">
 <tbody>
  <tr><td>foo</td><td>bar</td></tr>
 </tbody>
</table>

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