Fix passing relative path main jar to jpackage

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/desktop/application/internal/files/fileUtils.kt

@@ -108,5 +108,10 @@ internal fun File.checkExistingFile(): File =
 internal val File.isJarFile: Boolean
     get() = name.endsWith(".jar", ignoreCase = true) && isFile
 
-internal fun File.normalizedPath() =
-    if (currentOS == OS.Windows) absolutePath.replace("\\", "\\\\") else absolutePath
+internal fun File.normalizedPath(base: File? = null): String {
+    val path = base?.let { relativeToOrNull(it)?.path } ?: absolutePath
+    return when (currentOS) {
+        OS.Windows -> path.replace("\\", "\\\\")
+        else -> path
+    }
+}

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/desktop/application/tasks/AbstractJPackageTask.kt

@@ -321,8 +321,8 @@ abstract class AbstractJPackageTask @Inject constructor(
 
             val mappedJar = libsMapping[launcherMainJar.ioFile]?.singleOrNull()
                 ?: error("Main jar was not processed correctly: ${launcherMainJar.ioFile}")
-            val mainJarRelative = (if (currentTarget.os == OS.Windows) "\\" else "/") + mappedJar.relativeTo(libsDir.ioFile).toString()
-            cliArg("--main-jar", mainJarRelative)
+            val mainJarPath = mappedJar.normalizedPath(base = libsDir.ioFile)
+            cliArg("--main-jar", mainJarPath)
             cliArg("--main-class", launcherMainClass)
 
             if (currentOS == OS.Windows) {