Debugger: Fix switching to previous mode on exit

This behavior was broken with refactoring done in commit
3b5ecac238. This has two main components:
1. Perspective::select() needs to call EngineManager::activateDebugMode()
   in order to save the previous mode.
2. The contents of the previous function
   DebuggerPluginPrivate::activatePreviousMode() was placed in
   EngineManager::deactivateDebugMode() and is called in
   doFinishDebugger().

Task-number: QTCREATORBUG-21415
Change-Id: Ibca188ba740027769c497e25ea695af8e218ea4e
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggerengine.cpp

@@ -418,6 +418,8 @@ public:
         m_watchHandler.cleanup();
         m_engine->showMessage(tr("Debugger finished."), StatusBar);
         m_engine->setState(DebuggerFinished); // Also destroys views.
+        if (boolSetting(SwitchModeOnExit))
+            EngineManager::deactivateDebugMode();
     }
 
     void scheduleResetLocation()

src/plugins/debugger/debuggermainwindow.cpp

@@ -26,6 +26,7 @@
 #include "debuggermainwindow.h"
 #include "debuggerconstants.h"
 #include "debuggerinternalconstants.h"
+#include "enginemanager.h"
 
 #include <coreplugin/actionmanager/actioncontainer.h>
 #include <coreplugin/actionmanager/actionmanager.h>
@@ -696,7 +697,7 @@ void Perspective::addWindow(QWidget *widget,
 
 void Perspective::select()
 {
-    ModeManager::activateMode(Debugger::Constants::MODE_DEBUG);
+    Debugger::Internal::EngineManager::activateDebugMode();
     if (Perspective::currentPerspective() == this)
         return;
     theMainWindow->d->selectPerspective(this);

src/plugins/debugger/enginemanager.cpp

@@ -421,6 +421,18 @@ void EngineManager::activateDebugMode()
     }
 }
 
+void EngineManager::deactivateDebugMode()
+{
+    if (ModeManager::currentModeId() == Constants::MODE_DEBUG && d->m_previousMode.isValid()) {
+        // If stopping the application also makes Qt Creator active (as the
+        // "previously active application"), doing the switch synchronously
+        // leads to funny effects with floating dock widgets
+        const Core::Id mode = d->m_previousMode;
+        QTimer::singleShot(0, d, [mode]() { ModeManager::activateMode(mode); });
+        d->m_previousMode = Id();
+    }
+}
+
 bool EngineManager::isLastOf(const QString &type)
 {
     int count = 0;

src/plugins/debugger/enginemanager.h

@@ -49,6 +49,7 @@ public:
     static void unregisterEngine(DebuggerEngine *engine);
     static void activateEngine(DebuggerEngine *engine);
     static void activateDebugMode();
+    static void deactivateDebugMode();
     static bool isLastOf(const QString &type);
 
     static QList<QPointer<DebuggerEngine> > engines();