Preview does not pick up an added or removed _brand.yml until restarted

[cderv]

A quarto preview session resolves _brand.yml once and caches the result for the lifetime of the process. Adding, removing, or changing a sibling _brand.yml while preview is running has no effect on the output until preview is stopped and started again.

Reproduction

Start from a single document with no _brand.yml next to it:

---
title: "Report"
format: typst
---

Voir la [documentation](https://example.com).

quarto preview report.qmd --to typst --no-watch-inputs --no-browser

Now add a _brand.yml in the same directory:

color:
  palette:
    imperial-red: "#BC1E22"
  primary: imperial-red

Trigger a re-render (request a render, or edit and save the document). The link keeps its default color. Stop preview and render again and the brand is applied — the link turns red. The reverse case behaves the same way: start with _brand.yml present, remove it, and the brand stays applied until preview is restarted.

This is also what shows up from the RStudio IDE. The Render button runs quarto preview ... --no-watch-inputs and routes the following renders through the same long-lived preview server, so creating a _brand.yml and clicking Render again produces no visible change.

Cause

The preview server keeps a single ProjectContext for the whole session. projectResolveBrand populates project.brandCache on the first resolve and returns it on every subsequent call without re-scanning for _brand.yml:

quarto-cli/src/project/project-shared.ts

Lines 586 to 634 in 16257ef

	if (fileName === undefined) {
	if (project.brandCache) {
	return project.brandCache.brand;
	}
	project.brandCache = {};
	let fileNames = [
	"_brand.yml",
	"_brand.yaml",
	"_brand/_brand.yml",
	"_brand/_brand.yaml",
	].map((file) => join(project.dir, file));
	const brand = (project?.config?.brand ??
	project?.config?.project.brand) as
	| boolean
	| string
	| {
	light?: string;
	dark?: string;
	};
	if (brand === false) {
	project.brandCache.brand = undefined;
	return project.brandCache.brand;
	}
	if (
	typeof brand === "object" && brand &&
	("light" in brand || "dark" in brand)
	) {
	project.brandCache.brand = {
	light: brand.light
	? await loadSingleBrand(resolveBrandPath(brand.light, project.dir))
	: undefined,
	dark: brand.dark
	? await loadSingleBrand(resolveBrandPath(brand.dark, project.dir))
	: undefined,
	enablesDarkMode: !!brand.dark,
	};
	return project.brandCache.brand;
	}
	if (typeof brand === "string") {
	fileNames = [join(project.dir, brand)];
	}
	for (const brandPath of fileNames) {
	if (!existsSync(brandPath)) {
	continue;
	}
	project.brandCache.brand = await loadUnifiedBrand(brandPath);
	}
	return project.brandCache.brand;

Re-renders invalidate the per-file fileInformationCache but never brandCache:

quarto-cli/src/command/preview/preview.ts

Lines 465 to 467 in 16257ef

	if (project?.fileInformationCache) {
	project.fileInformationCache.invalidateForFile(file);
	}

A separate quarto render process resolves brand correctly because it builds a fresh ProjectContext; only the long-lived preview process is affected. Single-file and project (_quarto.yml) previews are both affected since they share the same cache field, and the behavior is format-independent (projectResolveBrand takes no format).

Suggestion

We could give brandCache a fingerprint over the candidate brand paths (_brand.yml, _brand.yaml, _brand/_brand.yml, _brand/_brand.yaml) covering existence and modification time, and re-resolve when it changes — similar to the mtime guard already used for the full-markdown cache.