How do I handle mouse focus in fullscreen mode? Does it work differently?

Question

Ryan McCombe · Accepted Answer

Mouse focus behaves differently in fullscreen mode because the window takes up the entire screen and typically prevents other windows from appearing above it. Here's what you need to know:

Fullscreen vs Windowed Mode

When your window is in true fullscreen mode, mouse focus events work differently:

The window always has mouse focus since there's nowhere else for the mouse to go
SDL_WINDOWEVENT_ENTER and SDL_WINDOWEVENT_LEAVE events won't fire unless you move to another screen
The mouse can be trapped within the window bounds

Here's how to test for fullscreen state and handle focus accordingly:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5void HandleWindowEvent(SDL_WindowEvent& E,
6                       SDL_Window* Window) {
7  bool IsFullscreen = SDL_GetWindowFlags(Window)
8    & SDL_WINDOW_FULLSCREEN;
9
10  if (E.event == SDL_WINDOWEVENT_ENTER) {
11    if (!IsFullscreen) {
12      std::cout << "Window gained mouse focus\n";
13    }
14  } else if (E.event == SDL_WINDOWEVENT_LEAVE) {
15    if (!IsFullscreen) {
16      std::cout << "Window lost mouse focus\n";
17    }
18  }
19}
20
21int main(int argc, char** argv) {
22  SDL_Init(SDL_INIT_VIDEO);
23  Window GameWindow;
24
25  // Toggle fullscreen with F key
26  SDL_Event E;
27  while (true) {
28    while (SDL_PollEvent(&E)) {
29      if (E.type == SDL_KEYDOWN
30        && E.key.keysym.sym == SDLK_f) {
31        bool IsFullscreen = SDL_GetWindowFlags(
32            GameWindow.SDLWindow) &
33          SDL_WINDOW_FULLSCREEN;
34        SDL_SetWindowFullscreen(
35          GameWindow.SDLWindow,
36          IsFullscreen
37            ? 0
38            : SDL_WINDOW_FULLSCREEN);
39      } else if (E.type == SDL_WINDOWEVENT) {
40        HandleWindowEvent(E.window,
41                          GameWindow.SDLWindow);
42      }
43    }
44    GameWindow.Update();
45  }
46
47  SDL_Quit();
48  return 0;
49}

1Window gained mouse focus
2Window lost mouse focus

Handling Multiple Monitors

If your game supports multiple monitors, you'll need to handle cases where the user moves between screens:

1#include <SDL.h>
2#include <iostream>
3#include "Window.h"
4
5void MonitorMousePosition(SDL_Window* Window) {
6  int MouseX, MouseY;
7  SDL_GetGlobalMouseState(&MouseX, &MouseY);
8
9  int DisplayIndex{
10    SDL_GetWindowDisplayIndex(Window)};
11  SDL_Rect DisplayBounds;
12  SDL_GetDisplayBounds(DisplayIndex,
13                       &DisplayBounds);
14
15  bool MouseOnSameDisplay{
16    MouseX >= DisplayBounds.x &&
17    MouseX < DisplayBounds.x + DisplayBounds.w &&
18    MouseY >= DisplayBounds.y &&
19    MouseY < DisplayBounds.y + DisplayBounds.h
20  };
21
22  std::cout << "\nMouse on game display: "
23    << (MouseOnSameDisplay ? "yes" : "no");
24}
25
26int main(int argc, char** argv) {
27  SDL_Init(SDL_INIT_VIDEO);
28  Window GameWindow;
29  SDL_Event E;
30
31  while (true) {
32    while (SDL_PollEvent(&E)) {
33      if (E.type == SDL_MOUSEMOTION) {
34        MonitorMousePosition(
35          GameWindow.SDLWindow);
36      }
37    }
38    GameWindow.Update();
39  }
40
41  SDL_Quit();
42  return 0;
43}

1Mouse on game display: no
2Mouse on game display: yes
3Mouse on game display: yes

Remember that fullscreen behavior can vary between operating systems and even between fullscreen modes (exclusive vs borderless windowed). Always test your focus handling across different configurations.

Managing Mouse Focus with SDL2

Mouse Focus in Fullscreen Mode

Fullscreen vs Windowed Mode

Handling Multiple Monitors

Managing Mouse Focus with SDL2

Game Development with SDL2

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