perf(UI): optimize app initialization by parallelizing API calls and lazy loading components

[balazs-szucs]

Description of Changes

This pull request improves frontend performance and user experience by optimizing component loading and UI transitions, as well as enhancing app initialization efficiency. The most significant changes are the introduction of React lazy loading and Suspense for heavy components on the home page, targeted CSS transitions for smoother theme switching, and parallelizing critical API calls during app startup.

Performance and Loading Optimization:

• Switched to React lazy and Suspense for heavy components (ToolPanel, Workbench, QuickAccessBar, RightRail, FileManager, AppConfigModal) in HomePage.tsx, enabling code-splitting and faster initial load times. A LoadingFallback component is shown while these components load.

• Parallelized fetching of /api/v1/config/app-config and /api/v1/info/status in AppConfigProvider to reduce app initialization time and improve perceived performance.

UI and Theming Improvements:

• Replaced the global CSS * transition rule with a more targeted [class*="mantine-"] selector for theme-related transitions, preventing unwanted transitions on non-theme elements.

• Updated mantineTheme button styles to use more granular transition properties, resulting in smoother and more consistent visual feedback for theme changes and interactions.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

Translations (if applicable)

• I ran scripts/counter_translation.py

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have run task check to verify linters, typechecks, and tests pass
• I have tested my changes locally. Refer to the Testing Guide for more details.

[Copilot]

Pull request overview

This PR targets frontend perceived performance by reducing initial JS work via code-splitting on the Home page, smoothing theme-switch transitions, and shortening app bootstrap by fetching config alongside a status probe.

Changes:

• Lazy-loads several heavy HomePage components with React.lazy + Suspense and introduces a shared loading fallback.
• Updates AppConfigProvider to fetch app-config while also issuing a status request during initialization, and refactors “resolved” tracking to use a ref.
• Narrows theme-switch CSS transitions (and makes Mantine button transitions more granular); updates AppConfigContext tests to account for the extra request.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
frontend/src/core/theme/mantineTheme.ts	Replaces transition: all with targeted button transitions.
frontend/src/core/styles/theme.css	Replaces global * transition rule with a Mantine-class-targeted selector.
frontend/src/core/pages/HomePage.tsx	Converts several major components to lazy-loaded modules behind a Suspense boundary.
frontend/src/core/contexts/AppConfigContext.tsx	Refactors resolved-config tracking to a ref and adds config+status fetching.
frontend/src/core/contexts/AppConfigContext.test.tsx	Updates mocks/assertions to reflect the new status call and refetch behavior.

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[jbrunton96]

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[jbrunton96]

Screen.Recording.2026-05-01.at.15.16.10.mov

I appreciate this might be too nitpicky, but there's some funny loading behaviour in this PR with things moving about on reload. Tool list is the most obvious place that this appears. Can we do anything about this to make sure it's always the same size before and after the contents has actually loaded in?

[balazs-szucs]

I appreciate this might be too nitpicky, but there's some funny loading behaviour in this PR with things moving about on reload. Tool list is the most obvious place that this appears. Can we do anything about this to make sure it's always the same size before and after the contents has actually loaded in?

No, that is one of the main goals of the PRs, so it is not nitpicky. It does look a bit weird, i'll see what i can do about it.

[balazs-szucs]

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[balazs-szucs]

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[balazs-szucs]

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