Add app-mark icon to the app template and fix FontIconSource NaN crash

[niels9001]

Summary

Gives the app scaffolded by dotnet new reactorapp a title-bar app-mark icon — the one bit of modern-title-bar polish the template was missing — and fixes a framework bug in icon-source resolution that adding the icon surfaced.

The template already shipped a Mica backdrop, an extended/modern title bar, and an interactive Hello, {name}! greeting. This PR leaves all of that untouched and only adds the icon.

Linked issue

Fixes #854

What changed

Template — tools/Templates/templates/WinUIApp-CSharp/App.cs (one line + a comment)

Add a placeholder app-mark icon to the title bar: a Segoe Fluent Icons CircleRing glyph (\uEA3A). A comment points authors at .Icon("ms-appx:///Assets/AppIcon.ico") for when they add a real asset.

-        var titleBar = TitleBar("Company.ReactorApp1").Flex(shrink: 0);
+        var titleBar = TitleBar("Company.ReactorApp1")
+            .Icon(FontIcon("\uEA3A", "Segoe Fluent Icons"))
+            .Flex(shrink: 0);

Framework fix — src/Reactor/Core/V1Protocol/IconResolver.cs

Adding that icon hit a runtime crash. Reactor caught the exception and painted it into the window instead of rendering the app — so "just add a title-bar icon" didn't actually work before this fix.

• What happened: ResolveIconSource(IconData) built the FontIconSource for the icon-source slot with an object initializer that always set FontSize = fi.FontSize ?? double.NaN (and FontFamily = null! when unspecified). A FontIconSource whose FontSize is NaN is rejected by TitleBar.set_IconSource with ArgumentException: Value does not fall within the expected range (E_INVALIDARG). So any FontIcon placed in an IconSource slot (TitleBar, TabView, …) without an explicit size threw. The parallel IconElement path (CreateFontIcon) was already correct — it only assigns FontFamily/FontSize when specified — which is why the bug stayed hidden (existing icon tests always passed an explicit size and never assigned the result to a real control).
• How it's fixed: replaced the initializer with a new CreateFontIconSource(fi) helper that mirrors CreateFontIcon — Glyph always, FontFamily/FontSize only when specified — leaving them at their control defaults otherwise.

Test

• Add selftest regression PrivMount_TitleBarFontIconNoSize (ReconcilerBigCoverageFixtures.cs): resolve a no-size FontIcon and assign it to a real TitleBar.IconSource, asserting no ArgumentException. Fails without the fix, passes with it.

Test plan

• dotnet build Reactor.slnx — clean.
• Selftest: dotnet run --project tests\Reactor.AppTests.Host -p:Platform=x64 -- --self-test --filter "PrivateMountHotPaths" → ok PrivMount_TitleBarFontIconNoSize, # Total failures: 0.
• Manually scaffolded and ran the template app: modern title bar now shows the CircleRing app mark + title over the Mica surface, no error panel.

Risk / breaking changes

• Template: one added icon line; scaffolded apps now show an app mark in the title bar. No public API change, and nothing else about the template changes.
• Framework fix is a strict bug fix. FontIcons that specified a size are unchanged; ones that omitted a size now correctly inherit the control's default size instead of throwing. No regression for callers that already worked.

[Copilot]

Pull request overview

This PR improves the default dotnet new reactorapp WinUI template by adding a modern title-bar app-mark icon, and fixes a Reactor framework bug that caused FontIconSource creation to crash when FontSize was left unspecified (surfaced by the new template icon).

Changes:

• Template: add a placeholder Segoe Fluent Icons glyph (\uEA3A CircleRing) as the title-bar app-mark icon, with a comment pointing to a future asset-based icon.
• Framework: fix IconResolver.ResolveIconSource(FontIconData) so FontFamily/FontSize are only assigned when specified (avoids FontSize = NaN / null assignment patterns that TitleBar.IconSource rejects).
• Selftest: add a regression check that assigns a no-size FontIcon-derived IconSource to a real TitleBar.IconSource and asserts it doesn’t throw.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tools/Templates/templates/WinUIApp-CSharp/App.cs	Adds a placeholder title-bar app-mark icon to the scaffolded app template.
src/Reactor/Core/V1Protocol/IconResolver.cs	Fixes FontIconSource construction to avoid invalid default assignments (prevents TitleBar.IconSource crash).
tests/Reactor.AppTests.Host/SelfTest/Fixtures/ReconcilerBigCoverageFixtures.cs	Adds a selftest regression to ensure a sizeless FontIcon resolves and can be assigned to TitleBar.IconSource.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[azchohfi]

Automated PR review — app-mark icon + FontIconSource NaN fix

Ran the repo's multi-dimensional pr-review skill (security, correctness, api-ergonomics, alternative-solution, test-coverage, docs-and-samples, packaging) plus a cross-model check (GPT-5.5, high reasoning) over origin/main...HEAD.

Verdict: looks good to merge. The fix is clean and the right shape — extracting CreateFontIconSource to mirror the existing CreateFontIcon and only assigning FontFamily/FontSize when specified is exactly how the IconElement path already works, so the two projections are now consistent. No security, correctness, API, packaging, or alternative-solution concerns surfaced.

Good call adding a real-TitleBar regression selftest that asserts the assignment no longer throws — that's a non-vacuous guard (it fails if FontSize = double.NaN is restored). Cross-check note: the fixture doesn't force a layout/render pass, so it relies on TitleBar.set_IconSource validating synchronously at assignment (which matches the app mount path and the reported failure) — worth keeping in mind if that WinUI behavior ever changes.

Only two minor, non-blocking notes (inline below):

• M1 (medium, test-coverage): the new test only isolates the missing-FontSize half of the change; the null-FontFamily branch isn't exercised on its own.
• L1 (low, docs): the template comment names glyph U+EA3A as "CircleRing", which independent sources dispute.

Summary: Critical: 0 High: 0 Medium: 1 Low: 1

[azchohfi]

M1 (medium, test-coverage): This regression case pins the FontSize-unset path, but it passes a non-null FontFamily ("Segoe Fluent Icons"), so the other half of the change — the if (fi.FontFamily is not null) branch in CreateFontIconSource — is never isolated. The pre-fix code also set FontFamily = fi.FontFamily is null ? null! : ..., so a null family was a distinct (and also potentially control-rejected) path.

Consider an adjacent check with a null family, e.g. new FontIconData("\uEA3A", fontFamily: null, fontSize: 18) assigned to a fresh TitleBar, so a future regression in the FontFamily branch can't slip through. Not a blocker.

[azchohfi]

L1 (low, docs): The comment labels U+EA3A as "CircleRing", but that glyph name doesn't hold up — the Segoe Fluent Icons mapping for U+EA3A is reported elsewhere as something else (e.g. "ContactInfo"/"Movies" depending on source), so the name is at best ambiguous. Since the code only depends on the raw codepoint, safest to either drop the name and call it a "placeholder glyph", or confirm it against the official Segoe Fluent Icons list. Purely cosmetic.

[niels9001]

Thanks for the review! Addressed the nits in fcf221a:

• M1 / @copilot (test-coverage): The selftest now asserts ResolveIconSource returns a real FontIconSource (not just that TitleBar.IconSource assignment doesn't throw), and adds a second case with a null FontFamily (new FontIconData("\uEA3A", FontFamily: null, FontSize: 18)) so the if (fi.FontFamily is not null) branch of CreateFontIconSource is exercised in isolation. Both cases are checked via a shared TitleBarAcceptsFontIconSource helper.
• L1 (docs): Dropped the disputed "CircleRing" name for U+EA3A in the template comment — it's now just a "placeholder Segoe Fluent Icons glyph".

Verified: PrivMount_TitleBarFontIconNoSize → ok, # Total failures: 0.