Increment day only when selection not focused

Manual increment will be done only if the selection element is not
focused, otherwise it would be incremented by two days.

src/App.vue

@@ -6,7 +6,7 @@
       <option v-for="mensa in mensen" :value="mensa.url">{{mensa.name}}</option>
     </select>
 
-    Wann? <select v-model="when">
+    Wann? <select v-model="when" id="selWhen">
       <option v-for="day in days" :value="day" :key="day">{{dayOfWeek[day]}}</option>
     </select>
     <router-view></router-view>
@@ -30,7 +30,7 @@ export default {
   name: 'app',
   data() {
     return {
-      menu: {},
+      selWhen: {},
       mensen: Mensen,
       days: [1, 2, 3, 4, 5, 6],
       dayOfWeek: {
@@ -45,10 +45,10 @@ export default {
   },
   methods: {
     previousDay() {
-      this.when === 1 ? this.when = 6 : this.when--
+      this.when === 1 ? (this.when = 6) : this.when--
     },
     nextDay() {
-      this.when === 6 ? this.when = 1 : this.when++
+      this.when === 6 ? (this.when = 1) : this.when++
     }
   },
   filters: {
@@ -86,26 +86,38 @@ export default {
       where: this.where
     })
     let vm = this
-    document.onkeydown = (evt) => {
+    document.onkeydown = evt => {
       evt = evt || window.event
       switch (evt.keyCode) {
         case 37:
+          if (vm.selWhen !== document.activeElement) {
+            // increment only if selection is not focused, avoid double execution
             vm.previousDay()
+          }
           break
         case 39:
+          if (vm.selWhen !== document.activeElement) {
+            // increment only if selection is not focused, avoid double execution
             vm.nextDay()
             break
           }
       }
     }
+  },
+  mounted() {
+    this.selWhen = document.getElementById('selWhen')
+  }
 }
 </script>
 
-<style src="./assets/base.css"></style>
-<style src="./assets/links.css"></style>
+<style src="./assets/base.css">
+</style>
+<style src="./assets/links.css">
+</style>
 <style>
 #app {
-  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto,
+    Oxygen-Sans, Ubuntu, Cantarell, 'Helvetica Neue', sans-serif;
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
   text-align: center;