Fix composer cut crash + suppress third-party plugin Sentry noise

[bengotow]

Summary

Fixes two Sentry issues found in the 1.22.0 release:

MAILSPRING-CLIENT-42 — Composer cut crashes with "Cannot read properties of undefined (reading 'firstChild')" (17 users, 41 events)

Root cause: onCut passed the real next callback to onCopy, which called next() once internally. Then onCut called next() a second time. This double-invocation caused slate-react's cloneFragment to run twice: the first time worked fine, but on the second invocation the DOM selection had already been modified by the cut, so firstChild was undefined.

slate-react:1740 (cloneFragment)  ← crashes here (second invocation)
slate-react:2357 (onCut)
slate:11430 (next)                ← second next() call, from onCut
composer-editor.tsx:221 (onCopy)  → next()  ← first next() call
slate:11430 (next)
composer-editor.tsx:225 (onCut)   → this.onCopy(event, editor, next)
slate-react:3011 (onCut)

Fix: Pass a no-op callback to onCopy from onCut, then call next() exactly once in onCut. onCopy still sets the text/html and text/plain clipboard data; the single next() call lets Slate attach its fragment and perform the text deletion.

MAILSPRING-CLIENT-3 — "Failed to activate plugin mailspring-avatars" (35 users, 475 events)

Root cause: mailspring-avatars is a user-installed third-party plugin (not in internal_packages). It fails with Cannot read properties of undefined (reading 'string') during activation — likely because it relies on a deprecated API (e.g. React.PropTypes.string). We have no control over this plugin's code.

Fix: In package-manager.ts, only report activation failures to Sentry for built-in packages (those whose directory is under resourcePath). For user-installed plugins, the error is still logged to the console but no longer sent to Sentry, where it is not actionable.

Test plan

• Open the composer, write some text, select it, and press Ctrl+X (Cut) — text should be cut to clipboard without a crash
• Confirm Ctrl+C (Copy) still works normally
• Confirm paste from cut works correctly in both composer and other apps
• Install a broken third-party plugin and confirm its activation error appears in the DevTools console but NOT in Sentry

Sentry links

• https://foundry-376-llc.sentry.io/issues/MAILSPRING-CLIENT-42
• https://foundry-376-llc.sentry.io/issues/MAILSPRING-CLIENT-3

🤖 Generated with Claude Code

https://claude.ai/code/session_01HgZZbFxdx38Ngc9vpanNow

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Generated by Claude Code

[indent-staging]

PR Summary

Two unrelated stability fixes for the composer and the plugin loader. The composer Cut path was calling slate-react's middleware chain twice (once via onCopy's internal next() and once explicitly), causing cloneFragment to run a second time on an already-mutated DOM and crash with "Cannot read properties of undefined (reading 'firstChild')". Plugin activation failures from user-installed third-party plugins were being reported to Sentry even though they're not actionable for Mailspring maintainers.

• ComposerEditor.onCut now passes a no-op to this.onCopy(...) and calls next() exactly once so slate-react's cloneFragment + editor.delete() runs a single time.
• PackageManager.activatePackage only forwards activation errors to AppEnv.reportError when pkg.directory.startsWith(this.resourcePath) (built-in internal_packages); third-party plugin failures are logged via console.error only.
• Localized error message is hoisted into a local const message to avoid duplication across the two reporting branches.

Issues

No issues found.

CI Checks

All CI checks passed on commit 09cfd4d.

Custom Rules 3 rules evaluated, 3 passed, 0 failed

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