Platform: get EmscriptenApplication canvas target from Module['canvas'].

And properly account for -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR.
Module['canvas'] can be read even from code compiled with -s MODULARIZE
so it's a preferrable option to hardcoding it in Configuration. The
target strings in Emscripten depend on whether we're compiled with
DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR (see
https://github.com/emscripten-core/emscripten/pull/7977). This is now
detected and handled at runtime to prepend element IDs with and use
the correct window and document magic targets.

src/Magnum/Platform/EmscriptenApplication.cpp

@@ -43,6 +43,13 @@
 #include "Magnum/Platform/GLContext.h"
 #endif
 
+/** @todo drop once we don't support < 1.38.27 anymore */
+#ifndef EMSCRIPTEN_EVENT_TARGET_DOCUMENT
+#define EMSCRIPTEN_EVENT_TARGET_DOCUMENT reinterpret_cast<const char*>(1)
+#define EMSCRIPTEN_EVENT_TARGET_WINDOW reinterpret_cast<const char*>(2)
+#define EMSCRIPTEN_EVENT_TARGET_SCREEN reinterpret_cast<const char*>(3)
+#endif
+
 namespace Magnum { namespace Platform {
 
 namespace {
@@ -174,6 +181,36 @@ namespace {
 
         return Key::Unknown;
     }
+
+    std::string canvasId() {
+        #pragma GCC diagnostic push
+        #pragma GCC diagnostic ignored "-Wgnu-zero-variadic-macro-arguments"
+        char* id = reinterpret_cast<char*>(EM_ASM_INT({
+            return allocate(intArrayFromString(Module['canvas'].id), 'i8', ALLOC_NORMAL);
+        }));
+        #pragma GCC diagnostic pop
+        std::string str = id;
+        std::free(id);
+        return str;
+    }
+
+    bool checkForDeprecatedEmscriptenTargetBehavior() {
+        /* Emscripten 1.38.27 changed to generic CSS selectors from element IDs
+        depending on -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 being
+        set.
+        https://github.com/emscripten-core/emscripten/pull/7977
+        There is no simple way to check for compiler options so check
+        whether the new CSS selectors are being used. If so, it should find
+        canvas#[id] which is any canvas with the ID of Module.canvas.
+        The old target behavior will look for an element with id="canvas#[id]"
+        which could theoretically exist but that's highly unlikely. */
+        bool deprecated = true;
+        Vector2d tempSize;
+        if(emscripten_get_element_css_size(("canvas#" + canvasId()).data(), &tempSize.x(), &tempSize.y()) >= 0) {
+            deprecated = false;
+        }
+        return deprecated;
+    }
 }
 
 EmscriptenApplication::EmscriptenApplication(const Arguments& arguments): EmscriptenApplication{arguments, Configuration{}} {}
@@ -266,10 +303,19 @@ bool EmscriptenApplication::tryCreate(const Configuration& configuration) {
     }
     #endif
 
+    std::ostream* verbose = _verboseLog ? Debug::output() : nullptr;
+
+    _deprecatedTargetBehavior = checkForDeprecatedEmscriptenTargetBehavior();
+    if(_deprecatedTargetBehavior) {
+        Debug{verbose} << "Platform::EmscriptenApplication::tryCreate(): using old Emscripten target behavior";
+    }
+
+    _canvasTarget = (_deprecatedTargetBehavior ? "" : "#") + canvasId();
+
     _dpiScaling = dpiScaling(configuration);
     if(!configuration.size().isZero()) {
         const Vector2i scaledCanvasSize = configuration.size()*_dpiScaling;
-        emscripten_set_canvas_element_size("#canvas", scaledCanvasSize.x(), scaledCanvasSize.y());
+        emscripten_set_canvas_element_size(_canvasTarget.data(), scaledCanvasSize.x(), scaledCanvasSize.y());
     }
 
     setupCallbacks(!!(configuration.windowFlags() & Configuration::WindowFlag::Resizable));
@@ -327,6 +373,17 @@ bool EmscriptenApplication::tryCreate(const Configuration& configuration, const
     _devicePixelRatio = Vector2{Float(emscripten_get_device_pixel_ratio())};
     Debug{verbose} << "Platform::EmscriptenApplication: device pixel ratio" << _devicePixelRatio.x();
 
+    /* Find out which element target strings Emscripten expects. This depends on
+       the DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR compiler option.  */
+    _deprecatedTargetBehavior = checkForDeprecatedEmscriptenTargetBehavior();
+    if(_deprecatedTargetBehavior) {
+        Debug{verbose} << "Platform::EmscriptenApplication::tryCreate(): using old Emscripten target behavior";
+    }
+
+    /* Get the canvas ID from Module.canvas, either set by EmscriptenApplication.js
+       or overridden/manually set by the user. */
+    _canvasTarget = (_deprecatedTargetBehavior ? "" : "#") + canvasId();
+
     /* Get CSS canvas size and cache it. This is used later to detect canvas
        resizes in emscripten_set_resize_callback() and fire viewport events,
        because browsers are only required to fire resize events on the window
@@ -346,10 +403,10 @@ bool EmscriptenApplication::tryCreate(const Configuration& configuration, const
     }
     _dpiScaling = dpiScaling(configuration);
     const Vector2i scaledCanvasSize = canvasSize*_dpiScaling*_devicePixelRatio;
-    emscripten_set_canvas_element_size("#canvas", scaledCanvasSize.x(), scaledCanvasSize.y());
+    emscripten_set_canvas_element_size(_canvasTarget.data(), scaledCanvasSize.x(), scaledCanvasSize.y());
 
     /* Create WebGL context */
-    EMSCRIPTEN_WEBGL_CONTEXT_HANDLE context = emscripten_webgl_create_context("#canvas", &attrs);
+    EMSCRIPTEN_WEBGL_CONTEXT_HANDLE context = emscripten_webgl_create_context(_canvasTarget.data(), &attrs);
     if(!context) {
         /* When context creation fails, `context` is a negative integer
            matching EMSCRIPTEN_RESULT_* defines */
@@ -371,23 +428,14 @@ bool EmscriptenApplication::tryCreate(const Configuration& configuration, const
 
 Vector2i EmscriptenApplication::windowSize() const {
     Vector2d size;
-    /* Emscripten 1.38.27 changed to generic CSS selectors from element IDs
+    emscripten_get_element_css_size(_canvasTarget.data(), &size.x(), &size.y());
-       depending on -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 being
-       set (which we can't detect at compile time). Fortunately, using #canvas
-       works the same way both in the previous versions and the current one.
-       Unfortunately, this is also the only value that works the same way for
-       both. Further details at
-       https://github.com/emscripten-core/emscripten/pull/7977 */
-    /** @todo don't hardcode "#canvas" everywhere, make it configurable from outside */
-    emscripten_get_element_css_size("#canvas", &size.x(), &size.y());
     return Vector2i{Math::round(size)};
 }
 
 #ifdef MAGNUM_TARGET_GL
 Vector2i EmscriptenApplication::framebufferSize() const {
     Vector2i size;
-    /* See above why hardcoded */
+    emscripten_get_canvas_element_size(_canvasTarget.data(), &size.x(), &size.y());
-    emscripten_get_canvas_element_size("#canvas", &size.x(), &size.y());
     return size;
 }
 #endif
@@ -423,12 +471,11 @@ void EmscriptenApplication::swapBuffers() {
 /* Called from window resize event but also explicitly from
    setContainerCssClass() */
 void EmscriptenApplication::handleCanvasResize(const EmscriptenUiEvent* event) {
-    /* See windowSize() for why we hardcode "#canvas" here */
     const Vector2i canvasSize{windowSize()};
     if(canvasSize != _lastKnownCanvasSize) {
         _lastKnownCanvasSize = canvasSize;
         const Vector2i size = canvasSize*_dpiScaling*_devicePixelRatio;
-        emscripten_set_canvas_element_size("#canvas", size.x(), size.y());
+        emscripten_set_canvas_element_size(_canvasTarget.data(), size.x(), size.y());
         ViewportEvent e{event, canvasSize,
             #ifdef MAGNUM_TARGET_GL
             framebufferSize(),
@@ -454,11 +501,7 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
        changes. Better than polling for this change in every frame like
        Sdl2Application does, but still not ideal. */
     if(resizable) {
-        #ifdef EMSCRIPTEN_EVENT_TARGET_WINDOW
+        const char* target = _deprecatedTargetBehavior ? "#window" : EMSCRIPTEN_EVENT_TARGET_WINDOW;
-        const char* target = EMSCRIPTEN_EVENT_TARGET_WINDOW;
-        #else
-        const char* target = "#window";
-        #endif
         auto cb = [](int, const EmscriptenUiEvent* event, void* userData) -> Int {
             static_cast<EmscriptenApplication*>(userData)->handleCanvasResize(event);
             return false; /** @todo what does ignoring a resize event mean? */
@@ -466,23 +509,21 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
         emscripten_set_resize_callback(target, this, false, cb);
     }
 
-    /* See windowSize() for why we hardcode "#canvas" here */
+    emscripten_set_mousedown_callback(_canvasTarget.data(), this, false,
-
-    emscripten_set_mousedown_callback("#canvas", this, false,
         ([](int, const EmscriptenMouseEvent* event, void* userData) -> Int {
             MouseEvent e{*event};
             static_cast<EmscriptenApplication*>(userData)->mousePressEvent(e);
             return e.isAccepted();
         }));
 
-    emscripten_set_mouseup_callback("#canvas", this, false,
+    emscripten_set_mouseup_callback(_canvasTarget.data(), this, false,
         ([](int, const EmscriptenMouseEvent* event, void* userData) -> Int {
             MouseEvent e{*event};
             static_cast<EmscriptenApplication*>(userData)->mouseReleaseEvent(e);
             return e.isAccepted();
         }));
 
-    emscripten_set_mousemove_callback("#canvas", this, false,
+    emscripten_set_mousemove_callback(_canvasTarget.data(), this, false,
         ([](int, const EmscriptenMouseEvent* event, void* userData) -> Int {
             auto& app = *static_cast<EmscriptenApplication*>(userData);
             /* With DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR, canvasX/Y is
@@ -499,7 +540,7 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
             return e.isAccepted();
         }));
 
-    emscripten_set_wheel_callback("#canvas", this, false,
+    emscripten_set_wheel_callback(_canvasTarget.data(), this, false,
         ([](int, const EmscriptenWheelEvent* event, void* userData) -> Int {
             MouseScrollEvent e{*event};
             static_cast<EmscriptenApplication*>(userData)->mouseScrollEvent(e);
@@ -516,31 +557,31 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
        1.38.27 depending on -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1
        but we don't want to force this flag on the users so the behavior
        handles both. */
-    #ifdef EMSCRIPTEN_EVENT_TARGET_DOCUMENT
+    const char* keyboardListeningElement = reinterpret_cast<const char*>(EM_ASM_INT({
-    char* const keyboardListeningElement = reinterpret_cast<char*>(EM_ASM_INT({
         var element = Module['keyboardListeningElement'] || document;
 
-        if(element === document) return 1;
+        if(element === document) return 1; /* EMSCRIPTEN_EVENT_TARGET_DOCUMENT */
-        if(element === window) return 2;
+        if(element === window) return 2; /* EMSCRIPTEN_EVENT_TARGET_WINDOW */
         if('id' in element)
             return allocate(intArrayFromString(element.id), 'i8', ALLOC_NORMAL);
 
         return 0;
     }));
-    #else
-    char* const keyboardListeningElement = reinterpret_cast<char*>(EM_ASM_INT({
-        var element = Module['keyboardListeningElement'] || document;
-
-        if(element === document) element = {id: '#document'};
-        if(element === window) element = {id: '#window'};
-        if('id' in element)
-            return allocate(intArrayFromString(element.id), 'i8', ALLOC_NORMAL);
-
-        return 0;
-    }));
-    #endif
     #pragma GCC diagnostic pop
 
+    std::string keyboardListeningElementString;
+    if(keyboardListeningElement == EMSCRIPTEN_EVENT_TARGET_DOCUMENT) {
+        keyboardListeningElement = _deprecatedTargetBehavior ? "#document" : keyboardListeningElement;
+    } else if(keyboardListeningElement == EMSCRIPTEN_EVENT_TARGET_WINDOW) {
+        keyboardListeningElement = _deprecatedTargetBehavior ? "#window" : keyboardListeningElement;
+    } else if(keyboardListeningElement) {
+        if(!_deprecatedTargetBehavior)
+            keyboardListeningElementString = "#";
+        keyboardListeningElementString += keyboardListeningElement;
+        std::free(const_cast<char*>(keyboardListeningElement));
+        keyboardListeningElement = keyboardListeningElementString.data();
+    }
+
     /* Happens only if keyboardListeningElement was set, but did not have an
        `id` attribute. Instead it should be either null or undefined, a DOM
        element, `window` or `document`. */
@@ -571,14 +612,6 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
             static_cast<EmscriptenApplication*>(userData)->keyReleaseEvent(e);
             return e.isAccepted();
         }));
-
-    #ifdef EMSCRIPTEN_EVENT_TARGET_DOCUMENT
-    if(keyboardListeningElement != EMSCRIPTEN_EVENT_TARGET_DOCUMENT &&
-       keyboardListeningElement != EMSCRIPTEN_EVENT_TARGET_WINDOW)
-    #endif
-    {
-        std::free(keyboardListeningElement);
-    }
 }
 
 void EmscriptenApplication::setupAnimationFrame(bool forceAnimationFrame) {

src/Magnum/Platform/EmscriptenApplication.h

@@ -896,6 +896,9 @@ class EmscriptenApplication {
         Flags _flags;
         Cursor _cursor;
 
+        bool _deprecatedTargetBehavior{};
+        std::string _canvasTarget;
+
         #ifdef MAGNUM_TARGET_GL
         EMSCRIPTEN_WEBGL_CONTEXT_HANDLE _glContext{};
         Containers::Pointer<Platform::GLContext> _context;