Fix: Construct 3 Physics Body Immovable Still Pushes on Collide

If you are searching for how to fix construct 3 physics body immovable still pushes on collide, you are not alone. This is a recurring issue in Construct 3 that comes up across many team projects. The behavior looks like a deep bug, but it usually traces back to a known interaction between two systems. Here is the full breakdown of the symptom, the cause, and a fix you can apply today.

The Symptom

You set a Physics body to Immovable so platforms don't move when the player walks on them. But the platforms still subtly push the player on contact, especially at high speeds.

Root Cause

Immovable in Construct 3's Box2D-based physics means the body has infinite mass for collision response, but the contact solver still applies position-correction impulses to resolve interpenetration. The solver pushes both objects apart, even immovable ones.

The Fix

Step 1: On the platform's Physics behavior, set Density to 0 (not just Immovable=true). Density 0 makes the body truly static - the solver doesn't compute reactions for it.

// Event: On start of layout
// Action: Platform → Physics → Set density to 0
// Action: Platform → Physics → Set Immovable to true

Step 2: Use the Solid behavior instead of Physics for platforms that don't need physics simulation - Solid is cheaper and doesn't push.

// Alternative - use Solid for non-physics platforms
// Add Solid behavior, remove Physics behavior on static platforms
// Player Platform behavior collides with Solids by default

Step 3: If you need both physics and immovability, set the collision bits so the player doesn't collide with the platform but other physics objects do.

Why this happens

This bug class sits at the boundary between two Construct 3 subsystems. The first system reports success at its layer; the second system silently rejects or transforms the data. Without an error in the middle, the symptom appears only at the visible output - which is where you started debugging.

The fix above addresses the configuration mismatch at the boundary. Once the two systems agree on the data contract, the symptom disappears immediately. There is no underlying engine bug to file; the behavior is a documented (if obscure) consequence of how Construct 3 designed the interaction.

Verifying the fix

Reproduce the original symptom in isolation before applying the fix. If you cannot reliably reproduce, you cannot reliably verify - and you risk shipping a fix that addresses a different bug. Start with a minimal scene or scenario that triggers the issue every time, apply the change above, and run the same scenario at least three times to confirm the symptom is gone.

For shipping games, follow a staged rollout. Push the fix to 5-10% of players first, monitor the affected metric (crash rate, error log frequency, gameplay telemetry) for 24-48 hours, and expand only if the data confirms the fix without regressions. A staged rollout is cheap insurance against an interaction you did not anticipate.

Capturing the bug from players

The hardest part of fixing this kind of issue is getting a player report that includes enough context to reproduce. Most players describe the symptom in their own words and omit the build number, scene, or hardware that triggered it. Without those, you are guessing at the conditions.

A bug reporting SDK like Bugnet for Construct 3 captures the build SHA, scene name, recent logs, device specs, and a screenshot automatically whenever a player files a report. With that bundle attached, you can reproduce the bug locally instead of guessing - typically the difference between a one-day fix and a one-week investigation.

Edge cases to watch for

The same root cause can produce slightly different symptoms in adjacent systems. After fixing the case you found, spend thirty minutes searching your project for similar patterns - the same API called with different arguments, the same data flow with a different entity type, or the same lifecycle issue in a sibling module. Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

Pay extra attention to boundary conditions - the first frame, the last frame, zero or maximum values, and the transition between two states. These are where engines often have undocumented behavior, and where regression tests pay the highest dividend. A test that exercises the boundary catches the subtle regressions that look like new bugs but are really the original returning.

When to escalate

If you have applied the fix above and the symptom persists, the bug is likely in a different layer than this article addresses. Capture a video of the symptom, the exact reproduction steps, and the Construct 3 version. File a report on the official issue tracker with that bundle - the maintainers are responsive when the report is complete.

Before filing, search the existing issues for keywords related to your symptom. Many bug reports are duplicates of issues that have a workaround posted in the comments but no formal fix in the engine. Reading the existing thread often resolves the issue faster than a new report.

Check the boundary; the bug lives between systems.