diff --git a/platform/core-api/src/com/intellij/openapi/module/ModuleServiceManager.java b/platform/core-api/src/com/intellij/openapi/module/ModuleServiceManager.java
index ad7e3b7265e6..59e3027ea779 100644
--- a/platform/core-api/src/com/intellij/openapi/module/ModuleServiceManager.java
+++ b/platform/core-api/src/com/intellij/openapi/module/ModuleServiceManager.java
@@ -23,6 +23,18 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 /**
+ * Provide a module service if you need to can be used to store data associated with a module. Its implementation should be registered in plugin.xml:
+ * <pre>
+ * &lt;extensions defaultExtensionNs="com.intellij"&gt;
+ *   &lt;moduleService serviceInterface="qualified-interface-class-name"
+                    serviceImplementation="qualified-implementation-class-name"/&gt;
+ * &lt;/extensions&gt;
+ * </pre>
+ * Class is loaded and its instance is created lazily when {@link #getService(Module, Class)} method is called for the first time.
+ * <p/>
+ * If the service implementation class implements {@link com.intellij.openapi.components.PersistentStateComponent} interface its state will
+ * be persisted in the module configuration file.
+ *
  * @author yole
  */
 public class ModuleServiceManager {
diff --git a/platform/projectModel-api/src/com/intellij/openapi/roots/ModuleExtension.java b/platform/projectModel-api/src/com/intellij/openapi/roots/ModuleExtension.java
index 7f5a2319066c..83af5e35cde3 100644
--- a/platform/projectModel-api/src/com/intellij/openapi/roots/ModuleExtension.java
+++ b/platform/projectModel-api/src/com/intellij/openapi/roots/ModuleExtension.java
@@ -22,7 +22,11 @@ import org.jdom.Element;
 import org.jetbrains.annotations.NotNull;
 
 /**
- * Implement {@link com.intellij.openapi.components.PersistentStateComponent} to be serializable.
+ * Extend this class to provide additional module-level properties which can be edited in Project Structure dialog. For ordinary module-level
+ * properties use {@link com.intellij.openapi.module.ModuleServiceManager module service} instead.
+ * <p/>
+ * If the inheritor implements {@link com.intellij.openapi.components.PersistentStateComponent} its state will be persisted in the module
+ * configuration file.
  */
 public abstract class ModuleExtension implements Disposable {
   public static final ExtensionPointName<ModuleExtension> EP_NAME = ExtensionPointName.create("com.intellij.moduleExtension");
diff --git a/platform/script-debugger/backend/src/SuspendContext.kt b/platform/script-debugger/backend/src/SuspendContext.kt
index 1c9cb81129eb..39fc56e98e95 100755
--- a/platform/script-debugger/backend/src/SuspendContext.kt
+++ b/platform/script-debugger/backend/src/SuspendContext.kt
@@ -58,9 +58,6 @@ interface SuspendContext<CALL_FRAME : CallFrame> {
 
   val vm: Vm
     get() = throw UnsupportedOperationException()
-
-  val workerId: String?
-    get() = null
 }
 
 abstract class ContextDependentAsyncResultConsumer<T>(private val context: SuspendContext<*>) : Consumer<T> {
diff --git a/platform/script-debugger/backend/src/SuspendContextManagerBase.kt b/platform/script-debugger/backend/src/SuspendContextManagerBase.kt
index 992a02cdf92a..7df95c55e5ab 100644
--- a/platform/script-debugger/backend/src/SuspendContextManagerBase.kt
+++ b/platform/script-debugger/backend/src/SuspendContextManagerBase.kt
@@ -55,7 +55,7 @@ abstract class SuspendContextManagerBase<T : SuspendContextBase<CALL_FRAME>, CAL
       throw IllegalStateException("Expected $context, but another suspend context exists")
     }
     context.valueManager.markObsolete()
-    debugListener.resumed()
+    debugListener.resumed(context.vm)
   }
 
   override val context: SuspendContext<CALL_FRAME>?
diff --git a/platform/script-debugger/backend/src/org/jetbrains/debugger/DebugEventListener.java b/platform/script-debugger/backend/src/org/jetbrains/debugger/DebugEventListener.java
index 764091b57961..c32bf65f9410 100755
--- a/platform/script-debugger/backend/src/org/jetbrains/debugger/DebugEventListener.java
+++ b/platform/script-debugger/backend/src/org/jetbrains/debugger/DebugEventListener.java
@@ -32,8 +32,9 @@ public interface DebugEventListener extends EventListener {
   /**
    * Reports the virtual machine has resumed. This can happen
    * asynchronously, due to a user action in the browser (without explicitly resuming the VM through
+   * @param vm
    */
-  default void resumed() {
+  default void resumed(Vm vm) {
   }
 
   /**
diff --git a/platform/script-debugger/debugger-ui/src/DebugProcessImpl.kt b/platform/script-debugger/debugger-ui/src/DebugProcessImpl.kt
index 620d31075949..88ddcb3973b9 100644
--- a/platform/script-debugger/debugger-ui/src/DebugProcessImpl.kt
+++ b/platform/script-debugger/debugger-ui/src/DebugProcessImpl.kt
@@ -135,7 +135,7 @@ abstract class DebugProcessImpl<C : VmConnection<*>>(session: XDebugSession,
   }
 
   val XSuspendContext?.vm: Vm
-    get() = (this as? SuspendContextView)?.activeExecutionStack?.suspendContext?.vm ?: mainVm!!
+    get() = (this as? SuspendContextView)?.activeVm ?: mainVm!!
 
   override final fun startForceStepInto(context: XSuspendContext?) {
     isForceStep = true
diff --git a/platform/script-debugger/debugger-ui/src/SuspendContextView.kt b/platform/script-debugger/debugger-ui/src/SuspendContextView.kt
index 183adc41ffad..a81a30980381 100644
--- a/platform/script-debugger/debugger-ui/src/SuspendContextView.kt
+++ b/platform/script-debugger/debugger-ui/src/SuspendContextView.kt
@@ -16,6 +16,7 @@
 package org.jetbrains.debugger
 
 import com.intellij.icons.AllIcons
+import com.intellij.openapi.diagnostic.logger
 import com.intellij.ui.ColoredTextContainer
 import com.intellij.ui.SimpleTextAttributes
 import com.intellij.util.ui.UIUtil
@@ -30,8 +31,6 @@ import org.jetbrains.debugger.frame.CallFrameView
 import org.jetbrains.debugger.values.StringValue
 import java.util.*
 
-const val MAIN_LOOP_NAME = "main loop"
-
 /**
  * Debugging several VMs simultaneously should be similar to debugging multi-threaded Java application when breakpoints suspend only one thread.
  * 1. When thread is paused and another thread reaches breakpoint, show notification about it with possibility to switch thread.
@@ -41,46 +40,48 @@ const val MAIN_LOOP_NAME = "main loop"
  * 4. Stepping/releasing updates current thread icon and clears frame, but doesn't switch thread. To release other threads, user needs to
  *    select them firstly.
  */
-abstract class SuspendContextView(protected val debugProcess: MultiVmDebugProcess, protected val activeStack: ExecutionStackView) : XSuspendContext() {
+abstract class SuspendContextView(protected val debugProcess: MultiVmDebugProcess,
+                                  activeStack: ExecutionStackView,
+                                  @Volatile var activeVm: Vm)
+  : XSuspendContext() {
 
-  protected open val stacks: Array<out XExecutionStack>  by lazy {
+  private val stacks: MutableMap<Vm, ScriptExecutionStack> = Collections.synchronizedMap(LinkedHashMap<Vm, ScriptExecutionStack>())
+
+  init {
     val mainVm = debugProcess.mainVm
     val vmList = debugProcess.collectVMs
     if (mainVm != null && !vmList.isEmpty()) {
-      val list = ArrayList<XExecutionStack>()
-
       // main vm should go first
-      vmList.mapNotNullTo(list) {
+      vmList.forEach {
         val context = it.suspendContextManager.context
-        if (context == activeStack.suspendContext) {
-          activeStack
-        }
-        else {
-          createStackView(context, it.presentableName)
-        }
+
+        val stack: ScriptExecutionStack =
+          if (context == null) {
+            RunningThreadExecutionStackView(it)
+          }
+          else if (context == activeStack.suspendContext) {
+            activeStack
+          }
+          else {
+            logger<SuspendContextView>().error("Paused VM was lost.")
+            InactiveAtBreakpointExecutionStackView(it)
+          }
+        stacks[it] = stack
       }
-      list.toTypedArray()
     }
     else {
-      arrayOf(activeStack)
+      stacks[activeVm] = activeStack
     }
   }
 
-  private fun createStackView(context: SuspendContext<*>?, displayName: String): XExecutionStack {
-    return if (context == null) {
-      RunningThreadExecutionStackView(displayName)
-    }
-    else {
-      ExecutionStackView(context, activeStack.viewSupport, null, null, displayName)
-    }
-  }
+  override fun getActiveExecutionStack() = stacks[activeVm]
 
-  override fun getActiveExecutionStack() = activeStack
-
-  override fun getExecutionStacks(): Array<out XExecutionStack> = stacks
+  override fun getExecutionStacks(): Array<out XExecutionStack> = stacks.values.toTypedArray()
 
   fun evaluateExpression(expression: String): Promise<String> {
+    val activeStack = stacks[activeVm]!!
     val frame = activeStack.topFrame ?: return rejectedPromise("Top frame is null")
+    if (frame !is CallFrameView) return rejectedPromise("Can't evaluate on non-paused thread")
     return evaluateExpression(frame.callFrame.evaluateContext, expression)
   }
 
@@ -94,9 +95,57 @@ abstract class SuspendContextView(protected val debugProcess: MultiVmDebugProces
           resolvedPromise(value.valueString!!)
         }
       }
+
+  fun pauseInactiveThread(inactiveThread: ExecutionStackView) {
+    stacks[inactiveThread.vm] = inactiveThread
+  }
+
+  fun hasPausedThreads(): Boolean {
+    return stacks.values.any { it is ExecutionStackView }
+  }
+
+  fun resume(vm: Vm) {
+    val prevStack = stacks[vm]
+    if (prevStack is ExecutionStackView) {
+      stacks[vm] = RunningThreadExecutionStackView(prevStack.vm)
+    }
+  }
+
+  fun resumeCurrentThread() {
+    resume(activeVm)
+  }
+
+  fun setActiveThread(selectedStackFrame: XStackFrame?): Boolean {
+    if (selectedStackFrame !is CallFrameView) return false
+
+    var selectedVm: Vm? = null
+    for ((key, value) in stacks) {
+      if (value is ExecutionStackView && value.topFrame?.vm == selectedStackFrame.vm) {
+        selectedVm = key
+        break
+      }
+    }
+
+    val selectedVmStack = stacks[selectedVm]
+    if (selectedVm != null && selectedVmStack is ExecutionStackView) {
+      activeVm = selectedVm
+      stacks[selectedVm] = selectedVmStack.copyWithIsCurrent(true)
+
+      stacks.keys.forEach {
+        val stack = stacks[it]
+        if (it != selectedVm && stack is ExecutionStackView) {
+          stacks[it] = stack.copyWithIsCurrent(false)
+        }
+      }
+
+      return stacks[selectedVm] !== selectedVmStack
+    }
+
+    return false
+  }
 }
 
-class RunningThreadExecutionStackView(displayName: String) : XExecutionStack(displayName, AllIcons.Debugger.ThreadRunning) {
+class RunningThreadExecutionStackView(vm: Vm) : ScriptExecutionStack(vm, vm.presentableName, AllIcons.Debugger.ThreadRunning) {
   override fun computeStackFrames(firstFrameIndex: Int, container: XStackFrameContainer?) {
     // add dependency to DebuggerBundle?
     container?.errorOccurred("Frames not available for unsuspended thread")
@@ -105,12 +154,33 @@ class RunningThreadExecutionStackView(displayName: String) : XExecutionStack(dis
   override fun getTopFrame(): XStackFrame? = null
 }
 
-// icon ThreadCurrent would be preferred for active thread, but it won't be updated on stack change
+class InactiveAtBreakpointExecutionStackView(vm: Vm) : ScriptExecutionStack(vm, vm.presentableName, AllIcons.Debugger.ThreadAtBreakpoint) {
+  override fun getTopFrame(): XStackFrame? = null
+
+  override fun computeStackFrames(firstFrameIndex: Int, container: XStackFrameContainer?) {}
+}
+
+abstract class ScriptExecutionStack(val vm: Vm, displayName: String, icon: javax.swing.Icon): XExecutionStack(displayName, icon) {
+  override fun hashCode(): Int {
+    return vm.hashCode()
+  }
+
+  override fun equals(other: Any?): Boolean {
+    return other is ScriptExecutionStack && other.vm == vm
+  }
+}
+
+// TODO should be AllIcons.Debugger.ThreadCurrent, but because of strange logic to add non-equal XExecutionStacks we can't update icon.
+private fun getThreadIcon(isCurrent: Boolean) = AllIcons.Debugger.ThreadAtBreakpoint
+
 class ExecutionStackView(val suspendContext: SuspendContext<*>,
                          internal val viewSupport: DebuggerViewSupport,
                          private val topFrameScript: Script?,
                          private val topFrameSourceInfo: SourceInfo? = null,
-                         displayName: String = "") : XExecutionStack(displayName, AllIcons.Debugger.ThreadAtBreakpoint) {
+                         displayName: String = "",
+                         isCurrent: Boolean = true)
+  : ScriptExecutionStack(suspendContext.vm, displayName, getThreadIcon(isCurrent)) {
+
   private var topCallFrameView: CallFrameView? = null
 
   override fun getTopFrame(): CallFrameView? {
@@ -159,6 +229,12 @@ class ExecutionStackView(val suspendContext: SuspendContext<*>,
           container.addStackFrames(result, true)
         }
   }
+
+  fun copyWithIsCurrent(isCurrent: Boolean): ExecutionStackView {
+    if (icon == getThreadIcon(isCurrent)) return this
+
+    return ExecutionStackView(suspendContext, viewSupport, topFrameScript, topFrameSourceInfo, displayName, isCurrent)
+  }
 }
 
 private val PREFIX_ATTRIBUTES = SimpleTextAttributes(SimpleTextAttributes.STYLE_ITALIC, UIUtil.getInactiveTextColor())