Backends: OSX: Fix keys remaining stuck in pressed state when CMD-tab…

…bing to a different application. (ocornut#3832)

backends/imgui_impl_osx.mm

@@ -18,6 +18,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2021-04-19: Inputs: Added a fix for keys remaining stuck in pressed state when CMD-tabbing into different application.
 //  2021-01-27: Inputs: Added a fix for mouse position not being reported when mouse buttons other than left one are down.
 //  2020-10-28: Inputs: Added a fix for handling keypad-enter key.
 //  2020-05-25: Inputs: Added a fix for missing trackpad clicks when done with "soft tap".
@@ -30,12 +31,15 @@
 //  2018-11-30: Misc: Setting up io.BackendPlatformName so it can be displayed in the About Window.
 //  2018-07-07: Initial version.
 
+@class ImFocusObserver;
+
 // Data
 static CFAbsoluteTime g_Time = 0.0;
 static NSCursor*      g_MouseCursors[ImGuiMouseCursor_COUNT] = {};
 static bool           g_MouseCursorHidden = false;
 static bool           g_MouseJustPressed[ImGuiMouseButton_COUNT] = {};
 static bool           g_MouseDown[ImGuiMouseButton_COUNT] = {};
+static ImFocusObserver* g_FocusObserver = NULL;
 
 // Undocumented methods for creating cursors.
 @interface NSCursor()
@@ -45,6 +49,46 @@ + (id)_windowResizeNorthSouthCursor;
 + (id)_windowResizeEastWestCursor;
 @end
 
+// Utility functions
+static int mapCharacterToKey(int c)
+{
+    if (c >= 'a' && c <= 'z')
+        return c - 'a' + 'A';
+    if (c == 25) // SHIFT+TAB -> TAB
+        return 9;
+    if (c >= 0 && c < 256)
+        return c;
+    if (c >= 0xF700 && c < 0xF700 + 256)
+        return c - 0xF700 + 256;
+    return -1;
+}
+
+static void resetKeys()
+{
+    ImGuiIO& io = ImGui::GetIO();
+    memset(io.KeysDown, 0, sizeof(io.KeysDown));
+    io.KeyCtrl = io.KeyShift = io.KeyAlt = io.KeySuper = false;
+}
+
+
+@interface ImFocusObserver : NSObject
+
+- (void)onApplicationBecomeInactive:(NSNotification*)aNotification;
+
+@end
+
+@implementation ImFocusObserver
+
+- (void)onApplicationBecomeInactive:(NSNotification*)aNotification
+{
+    // Unfocused applications do not receive input events, therefore we must manually
+    // release any pressed keys when application loses focus, otherwise they would remain
+    // stuck in a pressed state. https://github.com/ocornut/imgui/issues/3832
+    resetKeys();
+}
+
+@end
+
 // Functions
 bool ImGui_ImplOSX_Init()
 {
@@ -122,12 +166,19 @@ bool ImGui_ImplOSX_Init()
         strcpy(s_clipboard.Data, string_c);
         return s_clipboard.Data;
     };
+
+    g_FocusObserver = [[ImFocusObserver alloc] init];
+    [[NSNotificationCenter defaultCenter] addObserver:g_FocusObserver
+                                             selector:@selector(onApplicationBecomeInactive:)
+                                                 name:NSApplicationDidResignActiveNotification
+                                               object:nil];
 
     return true;
 }
 
 void ImGui_ImplOSX_Shutdown()
 {
+    g_FocusObserver = NULL;
 }
 
 static void ImGui_ImplOSX_UpdateMouseCursorAndButtons()
@@ -187,26 +238,6 @@ void ImGui_ImplOSX_NewFrame(NSView* view)
     ImGui_ImplOSX_UpdateMouseCursorAndButtons();
 }
 
-static int mapCharacterToKey(int c)
-{
-    if (c >= 'a' && c <= 'z')
-        return c - 'a' + 'A';
-    if (c == 25) // SHIFT+TAB -> TAB
-        return 9;
-    if (c >= 0 && c < 256)
-        return c;
-    if (c >= 0xF700 && c < 0xF700 + 256)
-        return c - 0xF700 + 256;
-    return -1;
-}
-
-static void resetKeys()
-{
-    ImGuiIO& io = ImGui::GetIO();
-    for (int n = 0; n < IM_ARRAYSIZE(io.KeysDown); n++)
-        io.KeysDown[n] = false;
-}
-
 bool ImGui_ImplOSX_HandleEvent(NSEvent* event, NSView* view)
 {
     ImGuiIO& io = ImGui::GetIO();