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    Debugging Frame Reflow

    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

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

    Getting the log

    • Make sure that your build is a debug build (in short you need ac_add_options --enable-debug in your .mozconfig file).
    • Create a text file (for instance reflow_rules.txt).
    • Enter this line in the text file
      * 1
    • This should look like:
    • point the GECKO_DISPLAY_REFLOW_RULES_FILE environment variable to the path of the reflow_rules.txt file.
    • Run the debug build, saving stdout to a file.
      • On Mac this is accomplished with:
        $ env DYLD_LIBRARY_PATH="`pwd`/obj-ff-dbg/dist/" \
        ./obj-ff-dbg/dist/ > logfile.txt

      After loading your testcase, the log file will contain the promised information.

    Log File analysis

    The log file for a simple table like

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

    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 
         tblO 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 
         tblO 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:

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

    Here the UC shows that on initial reflow the available height for the outer table 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 have the NS_FRAME_OUTSIDE_CHILDREN flag set. 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
    posInline positionedInline
    canvas canvas
    root root
    scroll scroll
    caption tableCaption
    cell tableCell
    bcCell bcTableCell
    col tableCol
    colG tableColGroup
    tbl table
    tblO tableOuter
    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


    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 alignment errors as text is drawn on pixel boundaries.


    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.

    Original Document Information

    • Author(s): Bernd Mielke
    • Last Updated Date: December 4, 2004
    • Copyright Information: Portions of this content are © 1998–2007 by individual contributors; content available under a Creative Commons license | Details.


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    Contributors to this page: Kohei, Dtrebbien, DBaron
    Last updated by: Dtrebbien,