Enalysis

After a framework upgrade and a design-system migration, the theme stylesheets still held framework leftovers, unused classes and rules copied verbatim into both light and dark. In a global stylesheet, any deletion can silently break a screen nobody thinks to check.

• The refactor ran behind a computed-style regression harness: 114 test pages rendered from the real stylesheets in both themes, with a baseline of computed styles for 6,115 elements. The baseline was taken twice to prove it was deterministic.
• A rule counted as unused only if its class appeared nowhere in the source, including class names built at runtime from template strings. That guard saved status-badge styles a naive pass would have deleted.
• A cascade “move guard” hoisted identical light/dark rules into the shared file only when no rule of equal specificity could start winning by source order. It kept 318 rules where they were.
• Every step was re-diffed against the baseline, with the test suite and formatter run each time.

The three stylesheets went from 16,466 to 14,864 lines (−10%): 371 unused rules removed and 47 duplicates moved into the shared file, with 0 rendering differences after every step and all 572 tests passing. Earlier passes using the same method caught two regressions before they landed, and both were rolled back.

The app's look lived in about 15,000 lines of hand-maintained CSS across a shared file and two themes, and one legacy colour did double duty for links and buttons. The new design system required exact token values in both themes, with no rounded or in-between shades. Midway through, the target design itself changed.

• Set the bar: exact token values everywhere and identical semantics across themes.
• Had an AI agent build a scripted pipeline instead of hand-editing. It classifies colours by selector context (splitting the overloaded colour into link, control and focus roles), snaps neutrals and hues to tokens, and converts alpha colours to exact hex, including inside SVG data URIs.
• After two over-broad find-and-replaces broke working rules, every hand fix became an idempotent script. The whole recolour could then be re-run from a clean tree when the target changed.
• Verification: a same-colour text/background scanner, a cross-theme comparator, a WCAG contrast audit and live computed-style probes.

The cross-theme comparator caught about 110 declarations an earlier pass had pushed into the wrong hue family. The final state has zero same-colour collisions and zero contrast failures in both themes, and 1,886 colour literals collapsed into about 50 palette tokens.

Switching themes briefly flashed the framework's stock colours. Later, an embedded report showed a white canvas, but only in dark mode, and editing the report's theme made no difference.

• The flash took three rounds. Swapping the stylesheet's href left an unstyled gap. Toggling media attributes still flashed, because the head-management library recreated the link elements.
• The final fix: static theme links ordered to win the cascade, a pre-paint inline script that applies the saved theme and an effect that flips media on the existing elements, so a switch is a synchronous restyle with nothing to fetch.
• For the iframe, the first two plausible causes were real bugs but not the culprit. As hypotheses piled up, I called a revert to the known-good branch to re-baseline.
• The root cause: a dark colour-scheme set on the page to theme scrollbars. Chromium paints an opaque backdrop behind a cross-origin iframe whose colour scheme differs from its host page's.

Theme toggles are instant with no unstyled frame. One scoped colour-scheme declaration fixed the report and kept the themed scrollbars.

A recipient picker in a create/edit modal lagged badly. It mounted every option as a DOM node and rebuilt its options on every render, so typing anywhere in the modal re-rendered the whole list.

• A windowed menu list mounts only the visible rows plus a small overscan, with spacers holding the scroll height.
• Keyboard-focused options still scroll into view. Fixed a ping-pong bug where that effect ran before the list had been measured.
• Stable component maps and options memoized on their source list, applied to both modals that share the picker.

About 99% fewer mounted option nodes (every option down to ~20), and roughly 10–60 ms of main-thread work per keystroke.

The single-page app was two major versions behind on React, React Router and Bootstrap, and the upgrade broke routing, analytics, maps and much of the test suite.

• Staged it: Node and packages, then React 18, then React Router 6 (routes, redirects and navigation prompts rewritten), then Bootstrap 5 with its utility-class renames.
• Pulled an analytics package that was incompatible with the new router, then reintroduced analytics through a tag manager.
• Burned down failing tests step by step and marked two unreliable tests as skipped, visible in every run, instead of deleting them.
• Later did the equivalent for the backend: moved the API and shared libraries to .NET 8 and removed their use of deprecated framework APIs.

Most dependency-audit findings were cleared, and failing tests fell from 110 to 21, 17, 12, 3 and finally 0. The modernized stack carried the product through the following years of feature work.

Trend data came back in local field time, so charts could misplace or duplicate points around daylight-saving transitions, and assets without a configured time zone had no reliable conversion.

• Resolved each asset's time zone from its settings, falling back to a geographic lookup from its coordinates.
• Converted trend timestamps to UTC in the API, handling local times that don't exist (spring-forward gaps) and times that happen twice (fall-back overlaps).
• Pinned the edge cases with unit tests and refined the logic through review.

Trend data comes back in consistent UTC across DST transitions, and the edge cases are covered by tests.