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emscripten_request_fullscreen_strategy callback order #9097

Daft-Freak opened this issue Jul 27, 2019 · 1 comment


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commented Jul 27, 2019

With this test code:

#include <stdio.h>

#include <emscripten/emscripten.h>
#include <emscripten/html5.h>


EM_BOOL fullscreen_change(int eventType, const EmscriptenFullscreenChangeEvent *fullscreenChangeEvent, void *userData)
    is_fs = fullscreenChangeEvent->isFullscreen;

    printf("%s fullscreen\n", is_fs ? "entered" : "left");

    return 0;

EM_BOOL canvas_resize(int eventType, const void *reserved, void *userData)

    double css_w, css_h;
    emscripten_get_element_css_size("#canvas", &css_w, &css_h);

    printf("resized to %fx%f\n", css_w, css_h);

    return 0;

EM_BOOL key_down(int eventType, const EmscriptenKeyboardEvent *keyEvent, void *userData)
    if(keyEvent->keyCode == 0x46/*f*/) {
        else {
            EmscriptenFullscreenStrategy strategy;
            strategy.scaleMode = EMSCRIPTEN_FULLSCREEN_SCALE_STRETCH;
            strategy.canvasResolutionScaleMode = EMSCRIPTEN_FULLSCREEN_CANVAS_SCALE_STDDEF;
            strategy.filteringMode = EMSCRIPTEN_FULLSCREEN_FILTERING_DEFAULT;
            strategy.canvasResizedCallback = canvas_resize;
            strategy.canvasResizedCallbackUserData = NULL;

            emscripten_request_fullscreen_strategy("#canvas", 1, &strategy);

    return 1;

void loop() {}

int main(int argc, char** argv) {

    emscripten_set_canvas_element_size("#canvas", 800, 600);

    emscripten_set_fullscreenchange_callback(EMSCRIPTEN_EVENT_TARGET_DOCUMENT, NULL, 0, fullscreen_change);
    emscripten_set_keydown_callback(EMSCRIPTEN_EVENT_TARGET_WINDOW, NULL, 0, key_down);
    emscripten_set_main_loop(loop, 0, 1);

    return 0;

Pressing F, F (using emscripten_exit_fullscreen) results in:

resized to 1920.000000x1080.000000
entered fullscreen
resized to 1920.000000x1080.000000
left fullscreen

And pressing F, Esc (cancelling through the browser) results in:

resized to 1920.000000x1080.000000
entered fullscreen
left fullscreen
resized to 800.000000x600.000000

When using emscripten_exit_fullscreen, the resize callback gets called before restoring the old size. It looks like what happens is:

  • emscripten_exit_fullscreen calls .exitFullscreen
  • Then it calls the resize callback and resets __currentFullscreenStrategy
  • The fullscreen change happens and the fullscreenchange handler for restoreOldStyle gets called
  • The canvas style is restored, but __currentFullscreenStrategy is 0 so the resize callback doesn't get called.

Removing the if (__currentFullscreenStrategy.canvasResizedCallback) block in emscripten_exit_fullscreen seems to work, but I'm not sure if that would break something else.

See: emscripten-ports/SDL2#87


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commented Aug 1, 2019

Interesting. It may be worth looking at the fullscreen tests in the interactive test suite. Those may not be up to date, but maybe they'll show a possible problem with your proposed fix.

cc @juj who wrote most of that code.

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