Fix for weird Alt-Tab behaviour in Linux.

- An extraneous TAB key event was being generated
  - This is possibly an SDL bug in Linux only; it doesn't happen on other systems
  - I've contacted the SDL mailing list for more info, so this code may be temporary

Changes.txt

@@ -12,6 +12,15 @@
                                Release History
 ===========================================================================
 
+5.0.1 to 5.x: (xxxx yy, 2017)
+
+  * Fixed an annoying bug in Linux, where Alt-Tab'ing out of a window and
+    then back again would pass a 'Tab' key event to the app, which in
+    most cases would navigate to the next UI element.
+
+-Have fun!
+
+
 5.0 to 5.0.1: (July 23, 2017)
 
   * Fixed issues in keypad, Genesis and various other controllers that use
@@ -25,8 +34,6 @@
 
   * Codebase now uses C++14 features.
 
--Have fun!
-
 
 4.7.3 to 5.0: (July 16, 2017)
 

src/emucore/EventHandler.cxx

@@ -62,6 +62,7 @@ EventHandler::EventHandler(OSystem& osystem)
     myFryingFlag(false),
     myUseCtrlKeyFlag(true),
     mySkipMouseMotion(true),
+    myAltKeyCounter(0),
     myContSnapshotInterval(0),
     myContSnapshotCounter(0)
 {
@@ -246,6 +247,16 @@ void EventHandler::handleTextEvent(char text)
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 void EventHandler::handleKeyEvent(StellaKey key, StellaMod mod, bool state)
 {
+  // Swallow KBDK_TAB and KBDK_RETURN under certain conditions
+  // See commments on 'myAltKeyCounter' for more information
+#ifdef BSPF_UNIX
+  if(myAltKeyCounter > 1 && (key == KBDK_TAB || key == KBDK_RETURN))
+  {
+    myAltKeyCounter = false;
+    return;
+  }
+#endif
+
   bool handled = true;
 
   // Immediately store the key state
@@ -263,8 +274,16 @@ void EventHandler::handleKeyEvent(StellaKey key, StellaMod mod, bool state)
     }
     else
 #endif
-    if(key == KBDK_RETURN)
+    if(key == KBDK_TAB)
     {
+      // Swallow Alt-Tab, but remember that it happened
+      myAltKeyCounter = 1;
+      return;
+    }
+    else if(key == KBDK_RETURN)
+    {
+      // Swallow Alt-Enter, but remember that it happened
+      myAltKeyCounter = 1;
       myOSystem.frameBuffer().toggleFullscreen();
     }
     // These only work when in emulation mode
@@ -823,11 +842,18 @@ void EventHandler::handleSystemEvent(SystemEvent e, int, int)
   switch(e)
   {
     case EVENT_WINDOW_EXPOSED:
-        myOSystem.frameBuffer().update();
-        break;
+      myOSystem.frameBuffer().update();
+      break;
+
+    case EVENT_WINDOW_FOCUS_GAINED:
+      // Used to handle Alt-x key combos; sometimes the key associated with
+      // Alt gets 'stuck'  and is passed to the core for processing
+      if(myAltKeyCounter > 0)
+        myAltKeyCounter = 2;
+      break;
 #if 0
     case EVENT_WINDOW_MINIMIZED:
-        if(myState == S_EMULATE) enterMenuMode(S_MENU);
+      if(myState == S_EMULATE) enterMenuMode(S_MENU);
         break;
 #endif
     default:  // handle other events as testing requires

src/emucore/EventHandler.hxx

@@ -583,6 +583,19 @@ class EventHandler
     // state change; we detect when this happens and discard the event
     bool mySkipMouseMotion;
 
+    // Sometimes key combos with the Alt key become 'stuck' after the
+    // window changes state, and we want to ignore that event
+    // For example, press Alt-Tab and then upon re-entering the window,
+    // the app receives 'tab'; obviously the 'tab' shouldn't be happening
+    // So we keep track of the cases that matter (Alt-Tab and Alt-Enter)
+    // and swallow the event afterwards
+    // Basically, the initial event sets the variable to 1, and upon
+    // returning to the app (ie, receiving EVENT_WINDOW_FOCUS_GAINED),
+    // the count is updated to 2, but only if it was already updated to 1
+    // TODO - This may be a bug in SDL, and might be removed in the future
+    //        It only seems to be an issue in Linux
+    uInt8 myAltKeyCounter;
+
     // Used for continuous snapshot mode
     uInt32 myContSnapshotInterval;
     uInt32 myContSnapshotCounter;