Debugging Frame Reflow

by 3 contributors:

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.


Document Tags and Contributors

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