app,io: [js] change Shortcut key on macOS/iOS

Previously, the Shortcut key was hardcoded as ModCtrl. That
patches tries to identify the OS and change the key.

Fixes: https://todo.sr.ht/~eliasnaur/gio/624
Signed-off-by: Lucas Rodrigues <inkeliz@inkeliz.com>
Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/key/mod.go

@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-//go:build !darwin
-
+//go:build !darwin && !js
 package key
 
 // ModShortcut is the platform's shortcut modifier, usually the ctrl

io/key/mod_js.go

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package key
+
+import (
+	"strings"
+	"syscall/js"
+)
+
+// ModShortcut is the platform's shortcut modifier, usually the ctrl
+// modifier. On Apple platforms it is the cmd key.
+var ModShortcut = ModCtrl
+
+// ModShortcut is the platform's alternative shortcut modifier,
+// usually the ctrl modifier. On Apple platforms it is the alt modifier.
+var ModShortcutAlt = ModCtrl
+
+func init() {
+	nav := js.Global().Get("navigator")
+	if !nav.Truthy() {
+		return // Almost impossible to happen
+	}
+
+	platform := ""
+	if p := nav.Get("platform"); p.Truthy() {
+		platform = p.String()
+	}
+	platform = strings.ToLower(platform)
+
+	// Based on https://developer.mozilla.org/en-US/docs/Web/API/Navigator/platform#examples
+	for _, darwinPlatform := range []string{"mac", "iphone", "ipad", "ipod"} {
+		if strings.HasPrefix(platform, darwinPlatform) {
+			ModShortcut = ModCommand
+			ModShortcutAlt = ModAlt
+			return
+		}
+	}
+}