Confused about .vue extensions - "Unknown custom element: <topmenu>" - templates

Question

I try to get working the electron-vue boilerplate. After setting up the project everything works, but as I create a new .vue file (TopMenu.vue) I get:
vue.common.js?4eb4:2569 [Vue warn]: Unknown custom element: <topmenu> -
did you register the component correctly? For recursive components, make
sure to provide the "name" option. (found in component <landing-page>)
I use the exact syntax as the original .vue files which came with the boilerplate:
LandingPageVue.vue:
<style scoped>
img {
margin-top: -25px;
width: 450px;
}
</style>
<template>
<div>
<!-- <img src="./LandingPageView/assets/logo.png" alt="electron-vue"> -->
<h1>Welcome.</h1>
<topmenu></topmenu>
<current-page></current-page>
<versions></versions>
<links></links>
<div class="container">
</div>
</template>
<script>
import TopMenu from './LandingPageView/TopMenu'
import CurrentPage from './LandingPageView/CurrentPage'
import Links from './LandingPageView/Links'
import Versions from './LandingPageView/Versions'
export default {
components: {
TopMenu,
CurrentPage,
Links,
Versions
},
name: 'landing-page'
}
</script>
TopMenu.vue (my file):
<template>
<p>
TOPMENU
</p>
</template>
By the way, how the hack does <current-page></current-page> work (notice the "-" dash) if bellow it is declared without?

Answer 1

It's not working because you're not exporting anything in your vue file.
Try this in your TopMenu.vue file:
<template>
<p>
TOPMENU
</p>
</template>
<script>
export default {
}
</script>
Also change the html <topmenu></topmenu> to <top-menu></top-menu>
For your second question, HTML is case insensitive so your title case components wouldn't match with html tags. So Vue translates your title case components to a 'dash-case'.
From the documentation itself there's the explanation why:
Note that Vue does not enforce the W3C rules for custom tag names (all-lowercase, must contain a hyphen) though following this convention is considered good practice.
You can read more from the the docs