Fix KeysDown not being reset to 'false' when losing focus

SFML doesn't send "KeyReleased" event as GLFW and SDL do, so we need to handle FocusLost event to allow for alt-tabbing.
SFML doesn't send the event of "Alt" begin released, so ImGui-SFML thinks that it's still pressed.

Fixes #147

imgui-SFML.cpp

@@ -302,9 +302,15 @@ void ProcessEvent(const sf::Event& event) {
  }
  break;
  case sf::Event::KeyPressed: // fall-through
- case sf::Event::KeyReleased:
- io.KeysDown[event.key.code] = (event.type == sf::Event::KeyPressed);
- break;
+ case sf::Event::KeyReleased: {
+ int key = event.key.code;
+ IM_ASSERT(key >= 0 && key < IM_ARRAYSIZE(io.KeysDown));
+ io.KeysDown[key] = (event.type == sf::Event::KeyPressed);
+ io.KeyCtrl = event.key.control;
+ io.KeyAlt = event.key.alt;
+ io.KeyShift = event.key.shift;
+ io.KeySuper = event.key.system;
+ } break;
  case sf::Event::TextEntered:
  // Don't handle the event for unprintable characters
  if (event.text.unicode < ' ' || event.text.unicode == 127) {
@@ -329,9 +335,20 @@ void ProcessEvent(const sf::Event& event) {
  }
 
  switch (event.type) {
- case sf::Event::LostFocus:
+ case sf::Event::LostFocus: {
+ // reset all input - SFML doesn't send KeyReleased
+ // event when window goes out of focus
+ ImGuiIO& io = ImGui::GetIO();
+ for (int i = 0; i < IM_ARRAYSIZE(io.KeysDown); ++i) {
+ io.KeysDown[i] = false;
+ }
+ io.KeyCtrl = false;
+ io.KeyAlt = false;
+ io.KeyShift = false;
+ io.KeySuper = false;
+
  s_windowHasFocus = false;
-   break;
+ } break;
  case sf::Event::GainedFocus:
  s_windowHasFocus = true;
  break;
@@ -384,12 +401,6 @@ void Update(const sf::Vector2i& mousePos, const sf::Vector2f& displaySize, sf::T
  }
  }
 
- // Update Ctrl, Shift, Alt, Super state
- io.KeyCtrl = io.KeysDown[sf::Keyboard::LControl] || io.KeysDown[sf::Keyboard::RControl];
- io.KeyAlt = io.KeysDown[sf::Keyboard::LAlt] || io.KeysDown[sf::Keyboard::RAlt];
- io.KeyShift = io.KeysDown[sf::Keyboard::LShift] || io.KeysDown[sf::Keyboard::RShift];
- io.KeySuper = io.KeysDown[sf::Keyboard::LSystem] || io.KeysDown[sf::Keyboard::RSystem];
-
 #ifdef ANDROID
 #ifdef USE_JNI
  if (io.WantTextInput && !s_wantTextInput) {