client: Fix chat window disappear on mobile

The chat window appears when #show-chat is clicked. That click leads
to #left becoming visible, which contains chat elements. However when
mobile user taps #input, on-screen keyboard appears, which triggers
screen resize -> setViewportHeight() and the latter calls showVideo()
which hides the chat completely.

-> Fix that by doing showVideo() in setViewportHeight() only if we were
not in chat mode.

static/galene.js

@@ -533,7 +533,9 @@ function setViewportHeight() {
     document.documentElement.style.setProperty(
         '--vh', `${window.innerHeight/100}px`,
     );
+    if (!getVisibility('left')) {
         showVideo();
+    }
     // Ajust video component size
     resizePeers();
 }
@@ -577,6 +579,16 @@ function setVisibility(id, visible) {
         elt.classList.add('invisible');
 }
 
+/**
+ * getVisibility tells whether specified element is visible.
+ *
+ * @param {string} id
+ */
+function getVisibility(id) {
+    let elt = document.getElementById(id);
+    return !elt.classList.contains('invisible');
+}
+
 /**
  * Shows and hides various UI elements depending on the protocol state.
  */