On Rendering Diffs :: Pierre Computer Company

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On Rendering Diffs :: Pierre Computer Company























PIERRE COMPUTER COMPANY █

Posted on May 29, 2026 by @amadeus

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You open a pull request expecting to understand what changed.

For small and medium changes, everything works. The code is readable, the files are there,
you scroll around, add comments, and it's all pretty seamless.

Then you open something larger. Maybe an agent generated the implementation, tests,
fixtures, and snapshots. Maybe the branch just touched more files than expected. Either way,
the review surface starts to degrade. It might only show you one file at a time, or require
each file to be loaded separately before you can read it, or even make basic navigation feel
sluggish.

Some of these are reasonable trade-offs for genuinely hard problems. But they still have a
cost: reviewers feel the limits of the tool, and product teams have to build workarounds for
these limits.

Diff rendering matters, but for most tools it is not the product. The product is what
happens around the code: review workflows, automation, agent output, CI results, and
collaboration. Code review should support that work, not become something every team has to
build from scratch.

That is why, about 6 months ago, we released
Diffs. Our goal was to make
the code and diff rendering part just work, so teams could spend their time on the product
around it.

Originally we launched with just the basic pieces: File and
FileDiff components. We quickly got feedback about performance issues, so we
followed up with a simple virtualizer that avoided rendering code when it was out of view
and an API to move syntax highlighting into worker threads. The simple virtualizer helped,
but it was a stopgap. There was still a lot of O(n×m) complexity, high memory usage, and virtualization blanking. What was missing was a
higher-level component that could manage an entire review surface and handle the hard
problems related to scale.

That missing layer became CodeView: a virtualization-first component for
reviewing code and diffs. And we built it around a deliberately impossible goal:

You should be able to just render any diff.

Not literally, of course. There are physical limits to browsers, compute, and memory. But
practically speaking, I think we've come pretty close, and I'd like to share a bit about how
we got there.

If you find long-form blog posts boring, go check out the CodeView playground
at DiffsHub.com where you can pretty
much view any PR or diff that GitHub will send our way. Nearly any diff, at any scale,
nearly instantly.

Take any public diff from GitHub and virtualize it nearly
instantly, no matter how large, with DiffsHub. Built to show off our brand new CodeView
component.

To try it out, replace `github` with `diffshub` in your address
bar. pic.twitter.com/5X30YwbpHn

- Pierre (@pierrecomputer)
May 20, 2026



You can check out the CodeView component and more in the latest version of the diffs package
on npm:
@pierre/diffs, or
read the docs.

DIFFS LOOK SIMPLE UNTIL THEY ARE NOT

On the surface, rendering diffs in a browser may not seem very hard. It's just text, right?
Browsers are purpose-built to take raw HTML and turn that into something you can look at and
interact with. Code is just text, after all.

But a good review surface needs more than text. It needs syntax highlighting, line numbers,
annotations, comments, theming, split and unified layouts, wrapping modes, and enough
customization to fit into someone else's product. Each of those features adds cost and
complexity. Syntax highlighting adds processing time and inflates DOM count. Comments
involve additional layout complexity that we can't fully control, and they still have to
work seamlessly with your existing design system.

With CodeView, we take that per-file complexity and scale it up; work that was
cheap for a single diff now has meaningful cost across a large review. We can roughly break
down the problems into three categories:

Rendering - DOM complexity grows quickly, and the browser can become
overloaded while scrolling or interacting with the page.

Processing - Every file or diff operation gets multiplied, so work that
was fast in isolation can become expensive when repeated thousands of times.

Memory - Large files and diffs get transformed into rendering data
structures, which can push against browser memory limits and make garbage collection more
frequent.

Our simple virtualizer helped with some rendering problems, and moving highlighting off the
main thread helped with parts of the processing problem. But CodeView needed to
treat rendering, memory, and processing as connected parts of the same problem.

Virtualization, or windowing, is a way of tackling the rendering problem. In its simplest
form, the idea is to only render the part of the content near the viewport. As you scroll,
the virtualizer renders the new content coming into view and removes content that has moved
off screen.

Keeping the DOM small has a lot of benefits: lower memory usage, less layout work, less
paint work, and fewer elements for the browser to manage. The trade-off is that the
virtualizer has to estimate or measure how tall everything is, and it must coordinate those
changes dynamically.

One thing that adds to this complexity is that browsers generally manage scroll compositing
separately from JavaScript execution. This can help scrolling feel more responsive to user
interactions, but it also means that JavaScript can easily lag behind scroll updates. This
is often most noticeable when using the scrollbar to make large jumps or scrolling extremely
quickly - the virtualizer can't keep up and you'll scroll into blank regions before the
JavaScript has time to render the updated content.

Click to see blanking in the old virtualizer

Common Virtualization Techniques

There are a few common ways to virtualize content in a browser, and each comes with its own
set of trade-offs.

The most common approach is to create a real scrollable region with the full estimated
height of the content, then position the visible items where they belong. This keeps
scrolling native: the scrollbar, momentum, input handling, and accessibility all stay with
the browser. The trade-off is that the rendered window can fall behind the visual scroll
position. Fast scrolls and large scrollbar jumps can expose blank space before JavaScript
has a chance to render the next range. You can reduce that by rendering a larger buffer
outside the viewport, but that gives back some of the DOM, layout, and memory savings that
virtualization was supposed to buy you.

Another approach is to keep the visible content in a sticky or fixed container and update
what it shows with requestAnimationFrame. In this model, blanking is
impossible: the content container cannot scroll out of view because it's not moving with the
scroll position; it just looks like it is. However, if JavaScript cannot keep up, then
scrolling can hitch or stutter because JavaScript is now part of the render update path.
Browser behavior matters here too. Safari, for example, currently caps
requestAnimationFrame at 60Hz even on higher refresh-rate displays, which makes
this approach feel worse than native scrolling on those devices.

A more extreme version is to emulate scrolling entirely: no native scrollable region, just a
custom viewport, a fake scrollbar, and content updated via
requestAnimationFrame as the user moves through the document. This can
avoid browser scroll-size limits because the scroll position is now your own state, not the
browser's. But the cost is larger: you now own the details of making scrolling feel native,
accessible, and correct across different operating systems and browsers.

The Inverse Sticky Technique

For CodeView, many of those virtualization trade-offs were not acceptable.
Native browser scrolling mattered. WebKit-based environments needed to feel good because
Tauri is a common target for developer tools. And blanking was not an option.

This left us stuck between different approaches that weren't quite right. After some
experimentation and frustration, we figured out a hybrid approach that could keep scrolling
native, mostly decouple positioning from requestAnimationFrame updates, and
make blanking effectively impossible.

We've called our new technique the Inverse Sticky Technique, but before we talk
about how it works, first a quick primer on how sticky positioning works. The
typical use case for sticky positioning is ensuring that section headers in a scrollable
list stay in view as you scroll through it. You set position: sticky; top: 0 on
your section headers and then when they should normally be scrolled out of view, they stay
fixed to the top of the scroll view as the content below scrolls underneath.

Section Title 1 (stuck)

Section Title 2 (stuck)

Section Title 3 (stuck)

For CodeView, we invert the usual sticky behavior. Instead of pinning the top
of the rendered content to the top of the viewport as you scroll down, the bottom edge of
the rendered region sticks to the bottom of the viewport when you scroll past it. When you
scroll back up, the top edge sticks to the top of the viewport.

This gives us native scrolling while the viewport is inside the rendered range. If
JavaScript falls behind, the rendered region sticks to one edge instead of scrolling away
and exposing blank space. We can get that behavior with negative top and
bottom sticky offsets, both calculated with the same formula:
(contentHeight - viewportHeight) * -1.

Here's a quick demo showing the Inverse Sticky Technique. We are
currently halfway scrolled down a larger scroll region.

Try scrolling up or down. This content scrolls with you until you hit the sticky
bounds, at which point this content will get stuck to the top or bottom.

So to circle back to the goals we set for ourselves: we preserve native scrolling, render
updates do not need to be frame-perfect to keep scrolling feeling smooth, and even large
jumps cannot scroll past the rendered content into blank space.

┌────────────────────────────────────────────────────┐
│ ┌────────────────────────────────────────────────┐ │
│ │ │ │

│ │ Full-height content element │ │
│ │ │ │

│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓ ▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓ Buffer element ▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓ before virtualized content ▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓ ▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ │ │
│ │ │ │

│ │ ┌────────────────────────────────────────────┐ │ │
│ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │
┌────────────────────────────────────────────────────────╖
│ ▀ ▀ ▀ ▓▓▓ Browser ▓▓▓ ║
├────────────────────────────────────────────────────────╢
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░ ░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░ Rendered content ░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░ ░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
│ │ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ │ ║
╘════════════════════════════════════════════════════════╝
│ │ │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│ │ |
│ │ └────────────────────────────────────────────┘ │ |
│ │ │ |
│ │ │ |
│ │ │ |
│ │ │ |
│ │ │ |
│ │ │ |
│ │ │ |
│ └────────────────────────────────────────────────┘ |
└────────────────────────────────────────────────────┘

While we were shooting for impossible to blank, Safari still found a way to
break our hearts. Under sufficiently aggressive scrolling, it can get backed up at the
compositing layer and expose blank space. It usually takes some work to pull off, but it is
still technically possible.

With virtualization in place, the next problem was calculating the layout and size of the
scrollable region. A virtualizer works best when its estimates are close to reality. Bad
estimates mean more corrective work after render: measuring DOM, updating item positions,
adjusting scroll height, and sometimes fixing the scroll offset to keep the current content
in place. The more often that happens, the more likely the page is to stutter or make the
scrollbar jump around.

Fortunately, the first pass is pretty cheap. Files are basically
lineHeight * totalLines. Diffs are only a little more complex because we
already have the parsed line counts and hunk metadata. From there, we just add the hunk
separators into the estimate. Simplified, it looks like this:
(lineHeight * diff.splitLineCount) + (diff.hunks.length * hunkSeparatorHeight).

Rendering Line Ranges

With our rough estimates in place, CodeView can determine which files should be
rendered. From there, each rendered file or diff gets the viewport size and position, and
uses that to decide which lines should be rendered internally.

This architecture came from the previous Virtualizer, but
CodeView pushed us to optimize some of the expensive paths. The old
implementation could end up iterating through a file or diff from the beginning to find
where the rendered range should start and end. For most files and diffs, that cost was
effectively invisible. But once we started testing much larger change sets, it became a
problem. A hunk with hundreds of thousands of lines could become pathologically expensive
because the lookup still had to start from zero.

To work around this, we added a cached position to line checkpoint system. That
lets us use binary search to find a closer starting point before doing the remaining range
search.

Once a line range is rendered, each file can verify its internal estimate against the actual
DOM and store any deltas. That lets the first-pass layout stay cheap while still correcting
the cases where the estimate was wrong.

Scroll anchoring is less about raw performance and more about keeping the view stable while
layout changes. If content above your scroll position changes height, the browser normally
tries to preserve what you were looking at instead of letting it jump around.

Browsers have built-in scroll anchoring for this, but virtualized views make that mostly
impossible. The mounted DOM is constantly changing, and the browser cannot make a safe
decision about which element to anchor to. For CodeView, we disable the
browser's built-in anchoring with overflow-anchor: none and handle it
ourselves.

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