• Summary
• Motivation
  • Goals
  • Non-Goals
  • Priority of Goals
• Proposal
  • Accessibility
    • ATAG 2.0 AA
• Design and implementation details
  • Workspace & Artifacts
  • Definitions
    • Maintainability
    • Done
  • Metrics
  • Front end
    • High-level implementation
    • HTML structure
    • Visual indicators
• Alternative Solutions
• Proposed changes
  • Front end (Design A)
    • High-level implementation
• Proposal (Design B)
• Design & Implementation Details (Design B)
  • Metrics
  • Overview
    • Static and dynamic separation
    • Performance optimizations

New diffs

Summary

Diffs at GitLab are spread across several places with each area using their own method. We are aiming to develop a single, performant way for diffs to be rendered across the application. Our aim here is to improve all areas of diff rendering, from the backend creation of diffs to the frontend rendering the diffs.

Motivation

Goals

• improved perceived performance
• improved maintainability
• consistent coverage of all scenarios

Non-Goals

Priority of Goals

In an effort to provide guidance on which goals are more important than others to assist in making consistent choices, despite all goals being important, we defined the following order.

Perceived performance is above improved maintainability is above consistent coverage.

Examples:

• a proposal improves maintainability at the cost of perceived performance: ❌ we should consider an alternative.
• a proposal removes a feature from certain contexts, hurting coverage, and has no impact on perceived performance or maintainability: ❌ we should re-consider.
• a proposal improves perceived performance but removes features from certain contexts of usage: ✅ it’s valid and should be discussed with Product/UX.
• a proposal guarantees consistent coverage and has no impact on perceived performance or maintainability: ✅ it’s valid.

In essence, we’ll strive to meet every goal at each decision but prioritise the higher ones.

Proposal

Accessibility

New diffs should be displayed in a way that is compliant with Web Content Accessibility Guidelines 2.1 level AA for web-based content and Authoring Tool Accessibility Guidelines 2.0 level AA for user interface.

We recognize that in order to have an accessible experience using diffs in the context of GitLab, we need to ensure the compliance both for displaying and interacting with diffs. That’s why the accessibility audit and further recommendation will also consider Content Editor used feature for reviewing changes.

ATAG 2.0 AA

Giving the nature of diffs, the following guidelines will be our main focus:

1. Guideline A.2.1: (For the authoring tool user interface) Make alternative content available to authors
2. Guideline A.3.1: (For the authoring tool user interface) Provide keyboard access to authoring features
3. Guideline A.3.4: (For the authoring tool user interface) Enhance navigation and editing via content structure
4. Guideline A.3.6: (For the authoring tool user interface) Manage preference settings

Design and implementation details

Workspace & Artifacts

• We will store implementation details like metrics, budgets, and development & architectural patterns here in the docs
• We will store large bodies of research, the results of audits, etc. in the wiki of the New Diffs project
• We will store audio & video recordings on the public YouTube channel in the Code Review / New Diffs playlist
• We will store drafts, meeting notes, and other temporary documents in public Google docs

Definitions

Maintainability

Maintainable projects are simple projects.

Simplicity is the opposite of complexity. This uses a definition of simple and complex described by Rich Hickey in “Simple Made Easy” (Strange Loop, 2011).

• Maintainable code is simple (single task, single concept, separate from other things).
• Maintainable projects expand on simple code by having simple structure (folders define classes of behaviors, e.g. you can be assured that a component directory will never initiate a network call, because that would be conflating visual display with data access)
• Maintainable applications flow out of simple organization and simple code. The old saying is a cluttered desk is representative of a cluttered mind. Rigorous discipline on simplicity will be represented in our output (the product). By being strict about working simply, we will naturally produce applications where our users can more easily reason about their behavior.

Done

GitLab has an existing definition of done which is geared primarily toward identifying when an MR is ready to be merged.

In addition to the items in the GitLab definition of done, work on new diffs should also adhere to the following requirements:

• Meets or exceeds all metrics
  • Meets or exceeds our minimum accessibility metrics (these are explicitly not part of our defined priorities, because they are non-negotiable)
• All work is fully documented for engineers (user documentation is a requirement of the standard definition of done)

Metrics

To measure our success, we need to set meaningful metrics. These metrics should meaningfully and positively impact the end user.

1. Meets or exceeds WCAG 2.2 AA.
2. Meets or exceeds ATAG 2.0 AA.
3. The new Diffs app loads less than or equal to 300 KiB of JavaScript (compressed / “across-the-wire”)¹.
4. The new Diffs app loads less than or equal to 150 KiB of markup, images, styles, fonts, etc. (compressed / “across-the-wire”)¹.
5. The new Diffs app can execute in total isolation from the rest of the GitLab product:
   1. “Execute” means the app can load, display data, and allows user interaction (“read-only”).
   2. If a part of the application is only used in merge requests or diffs, it is considered part of the Diffs application.
   3. If a part of the application must be brought in from the rest of the product, it is not considered part of the Diffs load (as defined in metrics 3 and 4).
   4. If a part of the application must be brought in from the rest of the product, it may not block functionality of the Diffs application.
   5. If a part of the application must be brought in from the rest of the product, it must be loaded asynchronously.
   6. If a part of the application meets 5.1-5.5 (such as: the Markdown editor is loaded asynchronously when the user would like to leave a comment on a diff) and its inclusion causes a budget overflow:
      • It must be added to a list of documented exceptions that we accept are out of bounds and out of our control.
      • The exceptions list should be addressed on a regular basis to determine the ongoing value of overflowing our budget.

¹: The Performance Inequality Gap, 2023

Front end

High-level implementation

HTML structure

The HTML structure of a diff should have support for assistive technology. For this reason, a table could be a preferred solution as it allows to indicate logical relationship between the presented data and is easier to navigate for screen reader users with keyboard. Labeled columns will make sure that information such as line numbers can be associated with the edited piece of code.

Possible structure could include:

<table>
  <caption class="gl-sr-only">Changes for file index.js. 10 lines changed: 5 deleted, 5 added.</caption>
  <tr hidden>
    <th>Original line number: </th>
    <th>Diff line number: </th>
    <th>Line change:</th>
  </tr>
  <tr>
    <td>1234</td>
    <td></td>
    <td>.tree-time-ago ,</td>
  </tr>
  […]
</table>

See WAI tutorial on tables for more implementation guidelines.

Each file table should include a short summary of changes that will read out:

• total number of lines changed,
• number of added lines,
• number of removed lines.

The summary of the table content can be placed either within <caption> element, or before the table within an element referred as aria-describedby. See WAI (Web Accessibility Initiative) for more information on both approaches:

• Nesting summary inside the <caption> element
• Using aria-describedby to provide a table summary

However, if such a structure will compromise other functional aspects of displaying a diff, more generic elements together with ARIA support can be used.

Visual indicators

It is important that each visual indicator should have a screen reader text denoting the meaning of that indicator. When needed, use gl-sr-only or gl-sr-only-focusable class to make the element accessible by screen readers, but not by sighted users.

Some of the visual indicators that require alternatives for assistive technology are:

• + or red highlighting to be read as added
• - or green highlighting to be read as removed

Alternative Solutions

Proposed changes

These changes (indicated by an arbitrary name like “Design A”) suggest a proposed final path forward for this blueprint, but have not yet been accepted as the authoritative content.

• Mark the highest hierarchical heading with your design name. If you are changing multiple headings at the same level, make sure to mark them all with the same name. This will create a high-level table of contents that is easier to reason about.

Front end (Design A)

Ideally, we would meet our definition of done and our accountability metrics on our first try. We also need to continue to stay within those boundaries as we move forward. To ensure this, we need to design an application architecture that:

1. Is:
   1. Scalable.
   2. Malleable.
   3. Flexible.
2. Considers itself a mission-critical part of the overall GitLab product.
3. Treats itself as a complex, unique application with concerns that cannot be addressed as side effects of other parts of the product.
4. Can handle data access/format changes without making UI changes.
5. Can handle UI changes without making data access/format changes.
6. Provides a hookable, inspectable API and avoids code coupling.
7. Separates:
   • State and application data.
   • Application behavior and UI.
   • Data access and network access.

High-level implementation

(See New Diffs: Technical Architecture Design for nicer visuals of this chart)

Proposal (Design B)

New diffs introduce a paradigm shift in our approach to rendering diffs. Previously, we had two different approaches to rendering diffs:

1. Merge requests heavily utilized client-side rendering.
2. All other pages used server-side rendering with sprinkles of JavaScript.

In merge requests, most of the rendering work was done on the client:

• The backend would only generate a JSON response with diffs data.
• The client would be responsible for both drawing the diffs and reacting to user input.

This led to us adopting a virtualized scrolling solution for client-side rendering, which sped up drawing large diff file lists significantly.

Unfortunately, this came with downsides of a very high maintenance cost and constant bugs. The user experience also suffered because we couldn’t show diffs right away when you visited a page, and had to wait for the JSON response first. Lastly, this approach went completely parallel to the server-rendered diffs used on other pages, which resulted in two completely separate codebases for the diffs.

The new-diffs approach changes that by doing the following:

1. Stop using virtualized scrolling for rendering diffs.
2. Move most of the rendering work to the server.
3. Enhance server-rendered HTML on the client.
4. Unify diffs codebase across merge requests and other pages.

Design & Implementation Details (Design B)

Metrics

1. (no change)
2. (no change)
3. (no change)
4. (no change)
5. (no change)
6. When rendering diffs on the server:
   • The total server-rendered count should not exceed 5 files.
   • It should not try to render empty diffs. (It should render at least 1 file.)
   • The total lines of diff code rendered should not exceed 1000 lines.

Overview

New diffs introduce a change in responsibilities for both frontend and backend.

The backend will:

1. Prepare diffs data.
2. Highlight diff lines.
3. Render diffs as HTML.
4. Embed diffs metadata into the final response.

The frontend will:

1. Enhance existing and future diffs HTML.
2. Fetch and render additional diffs HTML that didn’t fit into the page document.

Static and dynamic separation

To achieve the separation of concerns, we should distinguish between static and dynamic UI on the page:

• Everything that is static should always be rendered on the server.
• Everything dynamic should be enhanced on the client.

As an example: a highlighted diff line doesn’t change with user input, so we should consider rendering it on the server.

Performance optimizations

To improve the perceived performance of the page we should implement the following techniques:

1. Limit the number of diffs rendered on the page at first.
2. Use HTML streaming to render the rest of the diffs.
   1. Use Web Components to hook into diff files appearing on the page.
3. Apply content-visibility whenever possible to reduce redraw overhead.
4. Render diff discussions asynchronously.