Plugin fixture test fails after gradlew clean: stale .intellijPlatform/sandbox state

Problem

An IntelliJ Platform plugin test that uses a fixture, such as myFixture.getElementAtCaret(), fails on one machine and passes on another with the same project. The failure stays after ./gradlew clean and after a rerun with --rerun-tasks.

Example from a Groovy plugin test project built with the IntelliJ Platform Gradle Plugin:

• The test compiles a library JAR once with Groovy 4 and once with Groovy 5, and resolves a call to a method of that JAR.
• With the Groovy 5 JAR, the call does not resolve to a PsiMethod, and Quick Documentation has no target. With the Groovy 4 JAR, the same call resolves.
• The failure occurs on IntelliJ IDEA platform 2025.3.6 and 2026.1.4. The same test passes on 2026.2.

The difference between the two JARs (for example, a missing synthetic method in the Groovy 5 bytecode) looks like the cause, but the failure comes from the test environment.

Cause

The IntelliJ Platform Gradle Plugin keeps the test sandbox of the IDE under test in <project-directory>/.intellijPlatform/sandbox/<plugin-name>/<IDE>-<version>/system-test. This directory is outside build/. ./gradlew clean does not remove it, and --rerun-tasks does not reset it either. State left in the sandbox from earlier runs, such as cached data of a dependency JAR that changed, can change the result of a test.

Resolution

1. In the project root directory, run ./gradlew clean.

2. Delete the sandbox directory:

    rm -rf .intellijPlatform/sandbox

3. Run the test again, for example:

    ./gradlew --no-configuration-cache -PplatformVersion=2026.1.4 <testTask> --tests <TestClass> --rerun-tasks --console=plain

Additional information

The JVM that runs the test is neither the shell JAVA_HOME nor the project Java toolchain. The IntelliJ Platform Gradle Plugin sets javaLauncher of the test task to the JetBrains Runtime bundled with the platform under test, and a custom Test task that inherits from it uses the same JVM. To see the JVM of a test run, filter the --info output:

 ./gradlew <testTask> --tests <TestClass> --rerun-tasks --info \
   | grep "Starting process 'Gradle Test Executor" | grep -oE "Command: [^ ]+"

Related issue: IDEA-391904