Platform: expose MouseButton4 and 5 in EmscriptenApplication.

And the spec is again a total mess, as those are listed in the singular
button field, but not the buttons bitmask.

src/Magnum/Platform/EmscriptenApplication.cpp

@@ -512,6 +512,10 @@ EmscriptenApplication::Pointer buttonToPointer(const std::int32_t button) {
             return EmscriptenApplication::Pointer::MouseMiddle;
         case 2:
             return EmscriptenApplication::Pointer::MouseRight;
+        case 3:
+            return EmscriptenApplication::Pointer::MouseButton4;
+        case 4:
+            return EmscriptenApplication::Pointer::MouseButton5;
     }
 
     /* W3C spec allows other, platform-specific buttons:
@@ -532,6 +536,19 @@ EmscriptenApplication::Pointers buttonsToPointers(const std::uint32_t buttons) {
         pointers |= EmscriptenApplication::Pointer::MouseMiddle;
     if(buttons & (1 << 1))
         pointers |= EmscriptenApplication::Pointer::MouseRight;
+    /* https://www.w3.org/TR/uievents/#dom-mouseevent-buttons doesn't list
+       those even though the X1 and X2 buttons from
+        https://www.w3.org/TR/uievents/#dom-mouseevent-button
+       don't have any matching value here. In addition to the order swap and
+       the spec trying to describe bit flags in a *really* roundabout and
+       complicated way, this isn't entirely surprising. Chrome reports the
+       extra buttons, and the bit flags match here as well, so assume that's
+       correct. Firefox doesn't report X1 and X2 at all, so they're not present
+       here either. */
+    if(buttons & (1 << 3))
+        pointers |= EmscriptenApplication::Pointer::MouseButton4;
+    if(buttons & (1 << 4))
+        pointers |= EmscriptenApplication::Pointer::MouseButton5;
     return pointers;
 }
 

src/Magnum/Platform/EmscriptenApplication.h

@@ -1057,7 +1057,13 @@ enum class EmscriptenApplication::Pointer: UnsignedByte {
     MouseMiddle = 1 << 1,
 
     /** Right mouse button */
-    MouseRight = 1 << 2
+    MouseRight = 1 << 2,
+
+    /** Fourth mouse button, such as wheel left */
+    MouseButton4 = 1 << 3,
+
+    /** Fourth mouse button, such as wheel right */
+    MouseButton5 = 1 << 4,
 };
 
 CORRADE_ENUMSET_OPERATORS(EmscriptenApplication::Pointers)

src/Magnum/Platform/Test/EmscriptenApplicationTest.cpp

@@ -73,6 +73,8 @@ static Debug& operator<<(Debug& debug, Application::Pointer value) {
         _c(MouseLeft)
         _c(MouseMiddle)
         _c(MouseRight)
+        _c(MouseButton4)
+        _c(MouseButton5)
         #undef _c
     }
 
@@ -113,6 +115,8 @@ Debug& operator<<(Debug& debug, Application::Pointers value) {
         Application::Pointer::MouseLeft,
         Application::Pointer::MouseMiddle,
         Application::Pointer::MouseRight,
+        Application::Pointer::MouseButton4,
+        Application::Pointer::MouseButton5,
     });
 }