Debugging Frame Reflow

# Debugging Frame Reflow HowTo

## General Overview

The frame reflow can be logged with the debug capabilities implemented in `nsFrame.cpp`. It provides the following information for each frame at the start of its reflow

- reflow reason
- available width, available height
- computed width, computed height
- the previous and the next frame in flow
- and a count number.

When the frame's reflow is finished the following information is displayed :

- reflow metric (desired) width, height
- max. element size (MES) width
- maximum Width
- frame status
- overflow area

## Getting the log

- Make sure that your build is a debug build.
- Create a text file (for instance `reflow_rules.txt`).
- Enter this line in the text file

      * 1

- Save the text file in the `%DIST%` directory
- Go to the `%DIST%` directory
- enter from command line: `set GECKO_DISPLAY_REFLOW_RULES_FILE=reflow_rules.txt`
- Run `viewer > logfile.txt` and load your testcase. The logfile will contain all the promised information.

## Log File analyis

The logfile for a simple table like

    <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
    <html>
    <head>
      <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
    </head>
    <body>
    <table width="100">
      <tbody>
        <tr>
          <td>foo</td>
        </tr>
      </tbody>
    </table>
    </body>
    </html>

will create the following log:

    VP 00B97C30 r=0 a=9180,4470 c=9180,4470 cnt=856
     scroll 00B97EE0 r=0 a=9180,4470 c=9180,4470 cnt=857
     scroll 00B97EE0 r=0 a=9180,4470 c=9180,4470 cnt=858
      canvas 00B97C6C r=0 a=9180,UC c=9180,4470 cnt=859
       area 02D7AFE4 r=0 a=9180,UC c=9180,UC cnt=860
        text 02D7B150 r=0 a=9180,UC c=UC,UC cnt=861
        text 02D7B150 d=0,0
        block 02D7B210 r=0 a=9180,UC c=8940,UC cnt=862
        block 02D7B210 d=8940,0
       area 02D7AFE4 d=9180,120
      canvas 00B97C6C d=9180,4470
     scroll 00B97EE0 d=9180,4470
     scroll 00B97EE0 d=9180,4470
    VP 00B97C30 d=9180,4470
    VP 00B97C30 r=1 a=9180,4470 c=9180,4470 cnt=863
     scroll 00B97EE0 r=1 a=9180,4470 c=9180,4470 cnt=864
     scroll 00B97EE0 r=1 a=9180,4470 c=9180,4470 cnt=865
      canvas 00B97C6C r=1 a=9180,UC c=9180,4470 cnt=866
       area 02D7AFE4 r=1 a=9180,UC c=9180,UC cnt=867
        block 02D7B210 r=1 a=9180,UC c=8940,UC cnt=868
         text 02D7B3F8 r=0 a=8940,UC c=UC,UC cnt=869
         text 02D7B3F8 d=0,0
         tblW 02D7B5F0 r=0 a=8940,UC c=0,0 cnt=870
          tbl 02D7B7EC r=0 a=8940,UC c=1500,UC cnt=871
           rowG 00B984A4 r=0 a=UC,UC c=UC,UC cnt=872
            row 02D7BAF8 r=0 a=UC,UC c=UC,UC cnt=873
             cell 02D7BC98 r=0 a=UC,UC c=UC,UC cnt=874
              block 02D7BCF8 r=0 a=UC,UC c=UC,UC cnt=875
               text 02D7BE84 r=0 a=UC,UC c=UC,UC cnt=876
               text 02D7BE84 d=300,285 me=300
              block 02D7BCF8 d=300,300 me=300
             cell 02D7BC98 d=330,330 me=330
            row 02D7BAF8 d=UC,330
           rowG 00B984A4 d=UC,330
           colG 02D7BFB0 r=0 a=UC,UC c=UC,UC cnt=877
            col 02D7C0D8 r=0 a=0,0 c=1500,UC cnt=878
            col 02D7C0D8 d=0,0
           colG 02D7BFB0 d=0,0
           rowG 00B984A4 r=2 a=1500,UC c=1500,UC cnt=879
            row 02D7BAF8 r=2 a=1500,UC c=1500,UC cnt=880
             cell 02D7BC98 r=2 a=1440,UC c=1410,UC cnt=881
              block 02D7BCF8 r=2 a=1410,UC c=1410,UC cnt=882
              block 02D7BCF8 d=1410,300
             cell 02D7BC98 d=1440,330
            row 02D7BAF8 d=1500,330
           rowG 00B984A4 d=1500,330
           colG 02D7BFB0 r=2 a=1500,UC c=1500,UC cnt=883
            col 02D7C0D8 r=0 a=0,0 c=1500,UC cnt=884
            col 02D7C0D8 d=0,0
           colG 02D7BFB0 d=0,0
          tbl 02D7B7EC d=1500,390
         tblW 02D7B5F0 d=1500,390
         text 02D7C130 r=0 a=8940,UC c=UC,UC cnt=885
         text 02D7C130 d=0,0
        block 02D7B210 d=8940,390
       area 02D7AFE4 d=9180,630
      canvas 00B97C6C d=9180,4470
     scroll 00B97EE0 d=9180,4470
     scroll 00B97EE0 d=9180,4470
    VP 00B97C30 d=9180,4470

The first line shows the reflow of the viewport (`VP`). This viewport has the address `00B97C30`. It is the initial reflow: `r=0`. Other reflow reasons are:

1 | incremental reflow
2 | resize reflow
3 | style change reflow
4 | dirty reflow.

The available width is 9180 twips. The available height is 4470 twips (`a=9180,4470`). The computed width is 9180 twips. The computed height is 4470 twips (`c=9180,4470`). The line count is 856 (`cnt=856`).

Below this is a line that reads:

`tblW 02D7B5F0 r=0 a=8940,UC c=0,0 cnt=870`

Here the `UC` shows that on initial reflow the available height for the table wrapper frame is unconstrained.

The table cell requires its children to compute the MES. It is reported back from the block as:

`block 02D7BCF8 d=300,300 me=300`

The block max. element size is 300 twips.

The second table reflow is started at

`rowG 00B984A4 r=2 a=1500,UC c=1500,UC cnt=879`

where the previous information is used. The block has been required to compute the max. element size only once and it reports now:

`block 02D7BCF8 d=1410,300`

The block shows the same address as the previous one.

Frames with children that overflow the parent will return `PR_TRUE` from `HasOverflowRect()`. For these frames the overflow area is displayed as `block 025ED8F0 d=8940,1020 o=(0,0) 9180 x 1020`. The overflow area is specified as (x,y) origin and width x height.

The reflow finishes at the same level where it started.

## Advanced reflow debugging

The previously described technique dumps the data for every frame. Sometimes the log is clearer if only the main frames are shown. The entries in the reflow log can be controlled on a frame level. For instance adding `text 0` to the rules in `reflow_rules.txt` would hide the text entries from the reflow. The display of the following frames can be turned on by adding a line with the frame name and `1` or turned off by adding a line with the frame name and `0`:

short name | layout tag
--- | ---
area | area
block | block
br | br
bullet | bullet
button | gfxButtonControl
hr | hr
frameI | htmlFrameInner
frameO | htmlFrameOuter
img | image
inline | inline
letter | letter
line | line
select | select
obj | object
page | page
place | placeholder
canvas | canvas
root | root
scroll | scroll
caption | tableCaption
cell | tableCell
bcCell | bcTableCell
col | tableCol
colG | tableColGroup
tbl | table
tblW | tableWrapper
rowG | tableRowGroup
row | tableRow
textCtl | textInput
text | text
VP | viewport

Once the problem is reduced to a single frame level, placing a breakpoint at `DisplayReflowEnterPrint` in `nsFrame.cpp` is a very efficient way to step through the reflow tree.

## Other reflow debug options

### GECKO_DISPLAY_REFLOW_FLAG_PIXEL_ERRORS

Setting this option via `set GECKO_DISPLAY_REFLOW_FLAG_PIXEL_ERRORS = 1` enables a verification for each coordinate value that the coordinates are aligned at pixel boundaries.

                  row 0268A594 r=0 a=UC,UC c=UC,20 cnt=870
    VALUE 20 is not a whole pixel
                   cell 0268A6C0 r=0 a=UC,UC c=UC,15 cnt=871
                    block 0268A764 r=0 a=UC,UC c=UC,UC cnt=872
                    block 0268A764 d=0,0 me=0
                   cell 0268A6C0 d=0,0 me=0
                  row 0268A594 d=UC,20
    VALUE 20 is not a whole pixel
                 rowG 0268A02C d=UC,695
    VALUE 695 is not a whole pixel

While unaligned values at the entrance of a frame reflow can be ignored, when they appear at the exit of a routine this can cause display errors like stray lines. OS2 is very vulnerable to pixel alignement errors as text is drawn on pixel boundaries.

### GECKO_DISPLAY_REFLOW_INDENT_UNDISPLAYED_FRAMES

Setting this option via `set GECKO_DISPLAY_REFLOW_INDENT_UNDISPLAYED_FRAMES = 1` will cause an advance of the indent even for frames which are blocked via the reflow rules file.