Fix: Construct 3 Loader Layout Stuck at 99%

Your Construct 3 game loads in 3 seconds on a laptop but 30 seconds on a mobile browser. The progress bar rockets to 99% in 2 seconds, then stalls for 28. The network tab shows all fetches finished. Something is running synchronously after download and blocking the layout transition.

The Usual Blockers

Audio decoding. Every preloaded sound decodes from compressed (OGG, M4A) to PCM in the main thread. A 30-second music loop takes a second or two to decode on a phone. Ten preloaded sounds can add 10+ seconds to the end of the loader.

Webfont loading. If your project uses a Google Font or custom @font-face, the browser fetches it on first use. Text rendered during the loader layout blocks on font load. Preload fonts before Construct starts.

Image decoding. Large PNGs decode asynchronously but some browsers (Safari in particular) force sync decode on first paint. Use decoded image formats like WebP or pre-decoded textures.

The Fix

Step 1: Defer non-critical audio. In the Sound object’s properties, uncheck Preload for SFX. Only preload the music track that plays during the loader itself and a few high-priority UI sounds. Everything else loads lazily on first use.

Step 2: Preload webfonts in index.html.

<link rel="preload" href="assets/myfont.woff2" as="font" crossorigin>

Step 3: Profile with DevTools. Open Performance, click Record, reload the page, stop recording after the loader finishes. Look for long main-thread tasks between the last fetch and the layout transition. The task label tells you what’s blocking.

Loader Layout Design

Keep the loader layout minimal. One sprite, one progress bar, one fade-out. Heavy art on the loader forces Construct to decode those images before the loader can even start drawing, which delays the visible progress display.

Verifying

With the fix, the progress bar should reach 100% and the layout should transition within 300 ms. If it still hangs, record Performance again and find the new blocker. Always measure on the slowest target device — budget mobile phones are where loader bugs live.

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In Construct 3, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Construct 3. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Construct 3-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Construct 3, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“The last 1% of a loader is where half the bugs hide. The network is fast; the main thread is slow. Profile, don’t guess.”

Related Issues

For broader mobile performance, see Construct 3 performance low FPS lag. For audio issues after tab switches, see Construct 3 audio silent after tab switch.

Never preload an audio asset unless it plays in the first five seconds of gameplay. Every preloaded sound delays the loader.