wasm: Don't grab menu bar item shortcuts on Mac

On Mac there's no concept of menu bar alt+key shortcuts. Therefore we
cannot expect users on Mac, even when in a web application, to be
familiar with those. Additionally, alt+key combinations yield special
characters on Mac so that is the behavior we should support in WASM apps
on Mac.

To disable the menu bar item shortcuts, a new platform style hint
UnderlineShortcut was created. Shortcut grabbing was disabled exactly
the same as on regular Mac builds, i.e. through
qt_set_sequence_auto_mnemonic.

Task-number: QTBUG-76587
Change-Id: Ice6ed123c01e46b58d6d2c3e639813161b5a9a40
Reviewed-by: Mikołaj Boc <Mikolaj.Boc@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/wasm/qwasmintegration.cpp

@@ -10,7 +10,7 @@
 #include "qwasmaccessibility.h"
 #include "qwasmservices.h"
 #include "qwasmoffscreensurface.h"
-
+#include "qwasmplatform.h"
 #include "qwasmwindow.h"
 #include "qwasmbackingstore.h"
 #include "qwasmfontdatabase.h"
@@ -32,6 +32,8 @@
 
 QT_BEGIN_NAMESPACE
 
+extern void qt_set_sequence_auto_mnemonic(bool);
+
 using namespace emscripten;
 
 using namespace Qt::StringLiterals;
@@ -81,6 +83,9 @@ QWasmIntegration::QWasmIntegration()
 {
     s_instance = this;
 
+    if (platform() == Platform::MacOS)
+        qt_set_sequence_auto_mnemonic(false);
+
     touchPoints = emscripten::val::global("navigator")["maxTouchPoints"].as<int>();
 
     // Create screens for container elements. Each container element will ultimately become a
@@ -226,10 +231,14 @@ QAbstractEventDispatcher *QWasmIntegration::createEventDispatcher() const
 
 QVariant QWasmIntegration::styleHint(QPlatformIntegration::StyleHint hint) const
 {
-    if (hint == ShowIsFullScreen)
+    switch (hint) {
+    case ShowIsFullScreen:
         return true;
-
-    return QPlatformIntegration::styleHint(hint);
+    case UnderlineShortcut:
+        return platform() != Platform::MacOS;
+    default:
+        return QPlatformIntegration::styleHint(hint);
+    }
 }
 
 Qt::WindowState QWasmIntegration::defaultWindowState(Qt::WindowFlags flags) const