Enable linuxX64 native target for Compose Multiplatform Core libraries

[Aaron Chancey (hi-mod)]

Proposed Changes

This pull request enables the linuxX64 compilation and publication target for the Compose Multiplatform Core libraries (ui, ui-text, ui-backhandler, foundation, foundation-layout, animation, animation-core, material, material-ripple, and material3).

The implementation connects the Compose UI rendering runtime to the native Linux platform via Skiko:

• ComposeWindow.linux.kt: Connects an X11 window to Skiko's SkiaLayer using a CanvasLayersComposeScene. It contains an X11 event pump that polls for window input events, handles window resize signals, and invokes redrawing on the canvas.
• Key Mapping: Translates X11 key symbols (keysyms) to androidx key codes in Key.linux.kt.
• Main Dispatcher Integration: Routes tasks in PostDelayedDispatcher to SkikoDispatchers.Main to handle the lack of Dispatchers.Main in Kotlin/Native on Linux.
• Platform Services: Adds basic actuals for locale matching, POSIX calendar date formatting, (X11 CLIPBOARD selection: TARGETS/UTF8_STRING/TEXT/STRING (X11 CLIPBOARD selection: TARGETS/UTF8_STRING/TEXT/STRING serving, XConvertSelection paste with STRING fallback, INCR not supported, and URI handling via xdg-open using POSIX system() calls.
• Multiplatform Demo Support: Added the :compose:mpp:linux-demo sample project configured for linuxX64 target. Added a native Linux entry point at compose/mpp/linux-demo/src/linuxMain/kotlin/Main.kt that creates a ComposeWindow to render a shared Material3 counter app.
• Build Infrastructure Resilience: Replaced getByName("iosSimulatorArm64") with a safe findByName("iosSimulatorArm64") check inside AndroidXForkTargetsExtensions.kt (for both buildSrc and buildSrc-fork).
• Conditional Demo Targets: Updated compose/mpp/demo/build.gradle.kts to conditionally register JS, Wasm, macOS, and iOS targets using platform-enable checks (enableJs(), enableWasmJs(), enableMac()) and configured their source sets only if they are active. This prevents dependency resolution errors (such as missing target variants of :lifecycle:lifecycle-common) when building/syncing with restricted target platforms.

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Testing

Local Compilation and Publishing

1. Compiled the local repository targets using the multiplatform compile tasks:

   ./gradlew :compose:ui:ui:compileKotlinLinuxX64
   ./gradlew :compose:foundation:foundation:compileKotlinLinuxX64
   ./gradlew :compose:material3:material3:compileKotlinLinuxX64

2. Published the compiled artifacts to the local Maven repository:

   ./gradlew publishComposeJbToMavenLocal -Pcompose.platforms=linuxX64

End-to-End Verification

Built and ran the demo application in the consumer repository against the local maven artifacts:

1. Compiled the demo app using:

   ./gradlew linkDebugExecutableLinuxX64

2. Launched the native binary:

   ./build/bin/linuxX64/debugExecutable/linux-kotlin-native-desktop.kexe

3. Verified the interface using manual inspection and automated scripting:
   • Captured the screen visual output using xwd and converted to PNG to verify rendering correctness.
   • Tested pointer click interaction using python-xlib xtest.fake_input (simulating button press and release events) to verify target click response.
   • Verified clean exit (status code 0) after sending a WM_DELETE_WINDOW client message to close the application window.

Standalone Demo Verification

Verified compiling and running the built-in multiplatform demo within this repository:

1. Compiled and linked the demo executable:

   ./gradlew :compose:mpp:linux-demo:linkDebugExecutableLinuxX64

2. Ran the executable directly:

   ./out/compose-multiplatform-core/compose/mpp/linux-demo/build/bin/linuxX64/debugExecutable/linux-demo.kexe

QA Verification

This change should be tested by QA. Verification steps:

1. Compile and link the built-in demo module:

   ./gradlew :compose:mpp:linux-demo:linkDebugExecutableLinuxX64

2. Run the resulting executable on a Linux desktop system under a standard X11 session:

   ./out/compose-multiplatform-core/compose/mpp/linux-demo/build/bin/linuxX64/debugExecutable/linux-demo.kexe

3. Confirm that the window opens, UI widgets (buttons, dividers, text fields) render, interactive events function, and the application terminates with exit code 0 when closing the window.

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Local Development Tips

Gradle Sync with Local Skiko Snapshots

When syncing the project in IntelliJ or via Gradle against a locally built Skiko snapshot (which typically only contains target variants for JVM and Linux), Gradle dependency resolution will try to download the snapshot versions for all targets (Wasm, JS, macOS, iOS).
To prevent these resolution errors, restrict the active target platforms by adding the following property to your user-level Gradle properties file (~/.gradle/gradle.properties):

androidx.enabled.kmp.target.platforms=-wasm,-js,-mac,-windows,-android_native

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Release Notes

Features - Desktop

• Added experimental native linuxX64 target support for Compose Multiplatform Core libraries.
  • Enabled the linuxX64 target in the core multiplatform Gradle configurations.
  • Implemented ComposeWindow using a native X11 window hosted on Skiko's SkiaLayer.
  • Integrated X11 key symbol mapping, event loops, i(X11 CLIPBOARD selection: TARGETS/UTF8_STRING/TEXT/STRING serving, and UriHandlers.
  • Added a new native Linux demo project (:compose:mpp:linux-demo) showing Material3 integration.
  • Added build resilience for developers building/syncing with a subset of KMP platforms.

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[Jake Wharton (JakeWharton)]

Seems like a mistake to build anything new on X11. I also don't think we should assume any compositor for the majority of UI.

[Aaron Chancey (hi-mod)]

That makes sense. I will work on supporting Wayland and being compositor agnostic. I think I chose X11 because Mint defaults to that. I tried using Wayland and my screen just blanks so I will need to figure that out.