Block Layout

This document attempts to describe how "block" layout works in the mozilla layout engine.

nsBlockFrame implements layout behavior that conforms to the CSS "display:block" and "display: list-item" layout. It has several responsibilities:

Line layout. The block is responsible for flowing inline elements into "lines" and applying all of the css behavior as one might expect, including line-height, vertical-align, relative positioning, etc.
Float management. The block is responsible for the reflow and placement of floating elements.
Child block management. Blocks can contain inline elements and block elements. Hence, blocks are responsible for reflowing child blocks. The majority of that logic has been split out into nsBlockReflowContext, but a fair amount remains here.
Supporting table reflow. The block has to carefully compute the "max-element-size" information needed by tables. Hence, any time changes are made here one should always run the table regression tests because the odds are you broke one of them!

The Big Picture for Block Reflow

The block frame uses a list of nsLineBox's to keep track of each "line" of frames it manages. There are two types of lines:

"inline" lines which contain only inline elements
"block" lines which contain exactly one block element

Each line has a "dirty" bit which indicates that it needs reflow. Reflow consists of identifying which lines need to be marked dirty and then reflowing all lines. For lines which are "clean" the reflow logic will endeavor to recover the state of reflow as if the line had been reflowed. This saves time and allows for a faster incremental reflow. For lines which are dirty, the line is reflowed appropriately.

The only special thing about incremental reflow command handling is that it marks lines dirty before proceeding, and keeps track of the child frame that is the next frame on the reflow command path.

Here is a list of the various classes involved in block layout:

nsBlockFrame

The primary culprit.

nsBlockReflowState

This helper class is used to augment the nsHTMLReflowState with other information needed by the block reflow logic during reflow. It is a temporary object that is designed to live on the processor stack and contains "running" state used by the blocks reflow logic.

nsBlockBandData

Another helper class that wraps up management of a space manager (nsISpaceManager, nsSpaceManager) and nsBandData. It also assists in management of floating elements. While nsSpaceManager is policy free, nsBlockBandData provides specific HTML and CSS policy.

nsBlockReflowContext

A helper class that encapsulates the logic needed to reflow a child block frame. This is used by the block code reflow a child block and to reflow floating elements (which are to be treated as blocks according to the CSS2 spec).

nsLineBox

A data class used to store line information for the block frame code. Each line has a list of children (though the frames are linked together across lines to maintain the sibling list for nsIFrame::FirstChild) and some other state used to assist in incremental reflow.

nsLineLayout

This class is the line layout engine. Its a passive entity in the sense that its the responsibility of the block/inline code to use the class (this is done so that the line layout engine doesn't have to manage child frame lists so that both nsBlockFrame and nsInlineFrame can use the class).

nsTextRun

This is a data class used to store text run information. Text runs are logically contiguous runs of text (they may or may not be structurally contiguous). The block frame stores a pointer to a list of nsTextRun's and during line layout provides the list to the nsLineLayout engine so that when text is reflowed the text layout code (nsTextFrame) can find related text to properly handle word breaking.

Frame construction methods

When the blocks child list is modified (AppendFrames, InsertFrames, RemoveFrame) the block code updates its nsLineBox list. Since each nsLineBox is typed (some are marked "inline" and some are marked "block"), the update logic maintains the invariant of "one block frame per block line".

When structural changes are made to the blocks children (append/insert/remove) the block code updates the line's and then marks the affected lines "dirty" (each nsLineBox has a dirty bit). After the structural changes are finished then the block will generate an incremental reflow command of type "ReflowDirty".

Line Layout

Line layout consists of the placement of inline elements on a line until there is no more room on the line. At that point the line is "broken" and continued on the next line. This process continues until all inline elements have been exhausted. The block code maintains a list of "nsLineBox"'s to facilitate this. These are used instead of frames because they use less memory and because it allows the block to directly control their behavior.

The helper class nsLineLayout provides the majority of the line layout behavior needed by the block.

The block does keep "text-run" information around for the nsLineLayout logic to use during reflow. Text runs keep track of logically adjacent pieces of text within a block. This information is essential for properly computing line and word breaking. Why? Well, because in html you can write something like this:

<p>I <b>W</b>as thinking one day</p>

Notice that the word "Was" is composed of two pieces of text, and that they do not have the same parent (content or frame). To properly reflow this and not break the word prematurely after the "W", the text-run information is used by the text frame code to "look ahead" and prevent premature breaking.

Lines also keep track of the type of "break" that occurred on the line. This is used, for example, to support html's "<br clear=left>" behavior.

Float Management

Since child block elements are containing blocks for floats, the only place where a block frame will see a float is as part of an inline line. Consequently, the nsLineBox will only keep track of floats on inline lines (saving storage for block lines).

The nsLineLayout class and the block frame cooperate in the management of floats. Since the frame construction code leaves a "placeholder" frame in-flow where the float was found, when nsLineLayout reflows a placeholder frame it knows to inform the block about it. That triggers the blocks "AddFloat" logic which then determines where the float should be placed (on the current line or below the current line).

The block frame uses the space manager to manage the effects of floats, namely the consumption of available space. For example, for a left aligned floating element, the inline elements must be placed to the right of the float. To simplify this process, the spacemanager is used to keep track of available and busy space. Floats when placed mark space as busy and the spacemanager will them compute the available space. Most of this logic is handled by the nsBlockReflowState which uses a helper class, nsBlockBandData, in concert with the space manager, to do the available space computations.

Child Block Placement

Child block reflow is done primarily by using the nsBlockReflowContext code. However, a key detail worth mentioning here is how margins are handled. When the nsHTMLReflowState was created, we placed into it the logic for computing margins, border and padding (among other things). Unfortunately, given the css rules for sibling and generational margin collapsing, the nsHTMLReflowState is unable to properly compute top and bottom margins. Hence, the block frame and the nsBlockReflowContext code perform that function. At the time that the nsBlockReflowContext was designed and implemented we thought that it could compute the top-margin itself and then proceed to place the child block element. However, that turned out to be wrong (oh well) because the correct available space isn't known until after the top margin is computed. Hence, there is some unfortunate duplication of reflow state calculations present in the block frame code.

Bullets

Another type of block frame is the "display: list-item". List-items use nsBulletFrame's to manage bullet reflow. However, the block is responsible for bullet placement. In most situations, the nsLineLayout class is used to do the placement. However, if the first effective child of the block is another block, then the block has to do the placement itself.

Blank lines

Because our content model contains as much of the original source documents content as possible, we end up with a lot of white space that ends up being compressed into nothingness. This white space ends up impacting this logic in several ways. For example:

In the content model for the above html, there is white space between the various block elements (some after the <div>, some after the first </p>, again after the second </p>).

For css margin collapsing to work properly, each of those instances of white space has to behave as if they didn't exist. Consequently, there is special logic in the inline line reflow code, and in the nsBlockReflowContext code and in the GetTopBlockChild method, to basically ignore such lines.

First-letter style

The block contributes, in a small way, to first-letter style reflow. The frame construction code is responsible for creating the list of child frames for all frames, including the block. It manages the creation of letter-frames, where appropriate, so that all the block has to do is reflow them almost normally like other inline frames.

There are two things different that the block does:

It is responsible for calling nsLineLayout::SetFirstLetterStyleOK
It is responsible for continuing to place frames on a line, even after a frame has said "it can't fit". Normally during inline reflow, if a frame comes back and says it can't fit, the block will end the line, push all remaining frames to the next line and pick up the reflow from there after making sure the frame that didn't fit is continued. For letter-frames, this would result in the first-letter being on one line with the remaining text on subsequent lines. Hence, the block code handles this special case.

First-line style

First-line is handled entirely by the frame construction code.