Platform: avoid GlfwApplication::viewportEvent() being fired too soon.

Fixed sad an nasty crashes with GLFW-based apps on Windows.

doc/changelog.dox

@@ -268,6 +268,10 @@ See also:
 -   Fixed broken @ref Platform::EmscriptenApplication mouse event coordinates
     when `-s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1` is enabled (see
     [mosra/magnum#408](https://github.com/mosra/magnum/issues/408))
+-   Due to some Windows-specific issue with GLFW,
+    @ref Platform::GlfwApplication::viewportEvent() could get fired already
+    during class construction, potentially causing crashes. This is now worked
+    around by ignoring the very first event.
 -   In 2019.01 @ref Magnum/Platform/Sdl2Application.h went through an include
     cleanup, removing 50k lines; but unfortunately we forgot to add back
     @cpp #include <SDL_main.h> @ce, causing iOS builds to fail to initialize.

src/Magnum/Platform/GlfwApplication.cpp

@@ -58,6 +58,18 @@ enum class GlfwApplication::Flag: UnsignedByte {
     TextInputActive = 1 << 1,
     #ifdef CORRADE_TARGET_APPLE
     HiDpiWarningPrinted = 1 << 2
+    #elif defined(CORRADE_TARGET_WINDOWS)
+    /* On Windows, GLFW fires a viewport event already when creating the
+       window, which means viewportEvent() gets called even before the
+       constructor exits. That's not a problem if the window is created
+       implicitly (because derived class vtable is not setup yet and so the
+       call goes into the base class no-op viewportEvent()), but when calling
+       create() / tryCreate() from user constructor, this might lead to crashes
+       as things touched by viewportEvent() might not be initialized yet. To
+       fix this, we ignore the first ever viewport event. This behavior was not
+       observed on Linux or macOS (and thus ignoring the first viewport event
+       there may be harmful), so keeping this Windows-only. */
+    FirstViewportEventIgnored = 1 << 2
     #endif
 };
 
@@ -582,18 +594,26 @@ void GlfwApplication::setupCallbacks() {
         static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window))->drawEvent();
     });
     #ifdef MAGNUM_TARGET_GL
-    glfwSetFramebufferSizeCallback(_window, [](GLFWwindow* const window, const int w, const int h) {
-        auto& app = *static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window));
-        ViewportEvent e{app.windowSize(), {w, h}, app.dpiScaling()};
-        app.viewportEvent(e);
-    });
+    glfwSetFramebufferSizeCallback
     #else
-    glfwSetWindowSizeCallback(_window, [](GLFWwindow* const window, const int w, const int h) {
+    glfwSetWindowSizeCallback
+    #endif
+    (_window, [](GLFWwindow* const window, const int w, const int h) {
         auto& app = *static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window));
+        #ifdef CORRADE_TARGET_WINDOWS
+        /* See the flag for details */
+        if(!(app._flags & Flag::FirstViewportEventIgnored)) {
+            app._flags |= Flag::FirstViewportEventIgnored;
+            return;
+        }
+        #endif
+        #ifdef MAGNUM_TARGET_GL
+        ViewportEvent e{app.windowSize(), {w, h}, app.dpiScaling()};
+        #else
         ViewportEvent e{{w, h}, app.dpiScaling()};
+        #endif
         app.viewportEvent(e);
     });
-    #endif
     glfwSetKeyCallback(_window, [](GLFWwindow* const window, const int key, int, const int action, const int mods) {
         auto& app = *static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window));
 

src/Magnum/Platform/Test/GlfwApplicationTest.cpp

@@ -107,6 +107,10 @@ GlfwApplicationTest::GlfwApplicationTest(const Arguments& arguments): Platform::
     conf.setWindowFlags(Configuration::WindowFlag::Resizable);
     if(!args.value("dpi-scaling").empty())
         conf.setSize({800, 600}, args.value<Vector2>("dpi-scaling"));
+    /* Creating the window in the constructor exhibits a Windows-specific GLFW
+       issue where viewportEvent() gets called even before constructor exits;
+       it's now worked around but the same problem might start occuring on
+       other platforms as well in the future */
     create(conf);
 
     /* For testing resize events */