Hacking Gecko or 1011020

For some time now I want to be able to contribute to native support of MathML at Gecko but

  1. I only know C (not C++),
  2. I only program command line interfaces (not graphical user interface),
  3. I never program a “graphical framework”, ...

Frédéric recommended that I start with Bug 1011020 because shouldn’t be very hard to fix this bug. So I decided to give it a try.

Reproducing the bug

The bug is about the bars of fractions, radicals etc be invisible at some zoom levels. For example, I had to zoom out three times (or to 67%) for reproduce the bug with the following example.


And if I zoom out another time (or to 50%) the bar is visible.


I already have compile Firefox in the past and this was easy this time.


If you are going to compile Firefox for the first time, take a look at https://developer.mozilla.org/en/docs/Simple_Firefox_build.

Trying GDB

Since I already program in C and GDB help me a lot when debugging I decided to try using it for Firefox. Unfortunately it didn’t go very good since Firefox was very slow and because of this I decided to go back and use printf.

Finding the write file

To work with the msqrt element I

$ ls layout/mathml | grep -P '(sqrt|root).*cpp'


$ grep -i pixel layout/mathml/nsMathMLmsqrtFrame.cpp $ grep -i bar layout/mathml/nsMathMLmsqrtFrame.cpp

Since didn’t find anything, I

$ grep -i pixel layout/mathml/nsMathMLmrootFrame.cpp
  nscoord oneDevPixel = aFontMetrics->AppUnitsPerDevPixel();
  nscoord onePixel = nsPresContext::CSSPixelsToAppUnits(1);
  printf("onePixel      = %d\n", onePixel);
  if (ruleThickness < onePixel) {
    ruleThickness = onePixel;
  // adjust clearance psi to get an exact number of pixels -- this
  nscoord delta = psi % onePixel;
    psi += onePixel - delta; // round up

and because of it I start reading layout/mathml/nsMathMLmrootFrame.cpp and adding some printf on it. Unfortunately no information was send to the terminal and I asked for help. Frédéric reply suggesting to look at layout/mathml/nsMathMLmencloseFrame.cpp that first didn’t make much sense to me but after I looked at the documentation was very clear to me since I can use menclose as a replacement for msqrt, e.g.



I should have look at layout/mathml/nsMathMLmsqrtFrame.h and find out that it is related with layout/mathml/nsMathMLmencloseFrame.cpp.

Then I added the printf at layout/mathml/nsMathMLmencloseFrame.cpp and started get some information (after I discovered that nscoord is int).

onePixel, mRuleThickness and mRadicalRuleThickness

Reading layout/mathml/nsMathMLmencloseFrame.cpp looks that the relevant variables was onePixel, mRuleThickness and mRadicalRuleThickness so I printed it on the terminal and take some notes.

Zoom Level Bar is visible onePixel mRuleThickness mRadicalRuleThickness
100% Yes 60 120 39
90% Yes 60 134 44
80% Yes 60 75 42
67% Yes 60 90 42
50% No 60 120 45
30% Yes 60 200 56

Right now I’m trying to find what this numbers have of special that could help me fixing the bug.