Platform: proper feature parity for GlfwApplication::ViewportEvent.

Not sure why I omitted this in dba35bac7a.

doc/changelog.dox

@@ -76,9 +76,11 @@ See also:
 @subsubsection changelog-latest-new-platform Platform libraries
 
 -   Added @ref Platform::AndroidApplication::framebufferSize(),
-    @ref Platform::AndroidApplication::dpiScaling() and
+    @ref Platform::AndroidApplication::dpiScaling(),
-    @ref Platform::AndroidApplication::ViewportEvent::dpiScaling() for
+    @ref Platform::AndroidApplication::ViewportEvent::dpiScaling(),
-    compatibility with other application implementations
+    @ref Platform::GlfwApplication::ViewportEvent::framebufferSize() and
+    @ref Platform::GlfwApplication::ViewportEvent::dpiScaling() to ensure
+    feature parity with @ref Sdl2Application
 -   Clarified HiDPI behavior of Android apps and mentioning
     @ref platforms-android-apps-manifest-screen-compatibility-mode "the necessary steps"
     in the docs

src/Magnum/Platform/GlfwApplication.cpp

@@ -459,7 +459,8 @@ void GlfwApplication::setupCallbacks() {
         static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window))->drawEvent();
     });
     glfwSetFramebufferSizeCallback(_window, [](GLFWwindow* const window, const int w, const int h) {
-        ViewportEvent e{{w, h}};
+        auto& app = *static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window));
+        ViewportEvent e{{w, h}, app.framebufferSize(), app.dpiScaling()};
         static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window))->viewportEvent(e);
     });
     glfwSetKeyCallback(_window, [](GLFWwindow* const window, const int key, int, const int action, const int mods) {

src/Magnum/Platform/GlfwApplication.h

@@ -987,6 +987,8 @@ class GlfwApplication::Configuration {
          * value. The `--magnum-dpi-scaling` command-line option has a priority
          * over any application-set value.
          * @see @ref setSize(const Vector2i&, const Vector2&)
+         * @todo change this on a DPI change event (GLFW 3.3 has a callback:
+         *  https://github.com/mosra/magnum/issues/243#issuecomment-388384089)
          */
         Vector2 dpiScaling() const { return _dpiScaling; }
 
@@ -1138,16 +1140,41 @@ class GlfwApplication::ViewportEvent {
         /**
          * @brief Window size
          *
+         * On some platforms with HiDPI displays, window size can be different
+         * from @ref framebufferSize(). See @ref Platform-GlfwApplication-dpi
+         * for more information.
          * @see @ref GlfwApplication::windowSize()
          */
         Vector2i windowSize() const { return _windowSize; }
 
+        /**
+         * @brief Framebuffer size
+         *
+         * On some platforms with HiDPI displays, framebuffer size can be
+         * different from @ref windowSize(). See
+         * @ref Platform-GlfwApplication-dpi for more information.
+         * @see @ref GlfwApplication::framebufferSize()
+         */
+        Vector2i framebufferSize() const { return _framebufferSize; }
+
+        /**
+         * @brief DPI scaling
+         *
+         * On some platforms moving an app between displays can result in DPI
+         * scaling value being changed in tandem with a window/framebuffer
+         * size. Simply resizing a window doesn't change the DPI scaling value.
+         * See @ref Platform-GlfwApplication-dpi for more information.
+         * @see @ref GlfwApplication::dpiScaling()
+         */
+        Vector2 dpiScaling() const { return _dpiScaling; }
+
     private:
         friend GlfwApplication;
 
-        explicit ViewportEvent(const Vector2i& windowSize): _windowSize{windowSize} {}
+        explicit ViewportEvent(const Vector2i& windowSize, const Vector2i& framebufferSize, const Vector2& dpiScaling): _windowSize{windowSize}, _framebufferSize{framebufferSize}, _dpiScaling{dpiScaling} {}
 
-        Vector2i _windowSize;
+        Vector2i _windowSize, _framebufferSize;
+        Vector2 _dpiScaling;
 };
 
 /**