We all believe that NGLayout will replace the current layout sources, sometime around the end of this year. Until then, though, there is much that can still be squeezed out of the existing layout engine. NGLayout will not be ready to be included into the mozilla source by the time we want to release 5.0, so we've got at least another major (and probably several maintenance) releases which will use the current layout engine.

Mariner is the code name for performance and stability enhancements to the layout/layers/style sheet code. The project changes the layout engine to make the document model persist across resizes; so, each resize in Mozilla causes a page reflow instead of a reload from the cache. Now that we have a persistent document model, we are working on implementing a DOM interface to it.

Performance enhancements

Mariner includes a lot of performance enhancements, mainly in three areas:

• Text layout: The algorithm used to find break positions within text was improved. Also, the break positions were stored in a break table that gets consulted for doing text reflow.

• Table layout: In table layout, each cell gets laid out onto a rectangle of infinite width/height in the first pass. Then, once all the minimum and maximum widths/heights of cells have been figured out, a second pass of table layout occurs that lays out each cell's contents into a bounded rectangle. In 4.0, the second pass of table layout would reparse and relayout the cell from the HTML tags contained within the cell. In mozilla, because of Mariner's persistent document model, the second pass iterates over the model and adjusts the position and dimensions of the cell's content to fit in the bounded rectangle. All the malloc overhead incurred in the first pass for creating the document model is not incurred in the second pass. This make overall table layout much faster.

• Resize/Reflow: Since resizes cause a page reflow rather than a reload from the cache, they are much faster.

Many bugs related to resizing pages that have dynamic content on them are fixed.

Mariner DOM

Our current goal is to implement enough of the W3C spec to enable content developers to do what they want to do now. There are probably lots of things (reflecting certain properties from the HTML) that will be hard or impossible to do in the context of the current layout engine - these things just won't be implemented.

The current work on the Mariner DOM is taking place on the "MARINER_DOM_BRANCH" branch of mozilla/include and mozilla/lib.

The code that's currently there for the DOM was more of a proof of concept than anything else. We reflected span elements into javascript, and allowed the setting of some interesting properties (font size, weight, face, text color, background color). After each style change, we reflowed the document using the mariner reflow call.

You can help (please :)

Please send mail to the addresses at the top of the page - we need people that aren't afraid to get their hands dirty working with some (admittedly) convoluted code. Help in multibyte text layout would be wonderful, and anyone willing to work on the DOM would be welcome as well. We've got some ideas on the way the DOM could be placed around the current layout source, and will be putting up a header file outlining it soon.

Given that there are only two of us (with a third coming on soon) there is plenty of room for others to help out and make a difference in the stability, performance, and feature set of 5.0.