1 files changed, 58 insertions, 2 deletions
diff --git a/ui/routes/(app)/ch/[channel]/+page.svelte b/ui/routes/(app)/ch/[channel]/+page.svelte
index 0961665..7d5b8cf 100644
--- a/ui/routes/(app)/ch/[channel]/+page.svelte
+++ b/ui/routes/(app)/ch/[channel]/+page.svelte
@@ -1,14 +1,70 @@
 <script>
+  import { DateTime } from 'luxon';
   import { page } from '$app/stores';
   import ActiveChannel from '$lib/components/ActiveChannel.svelte';
   import MessageInput from '$lib/components/MessageInput.svelte';
-  import { messages } from '$lib/store';
+  import { channelsList, messages } from '$lib/store';
 
   let channel = $derived($page.params.channel);
   let messageRuns = $derived($messages.inChannel(channel));
+  let activeChannel;
+
+  function inView(parentElement, element) {
+    const parRect = parentElement.getBoundingClientRect();
+    const parentTop = parRect.top;
+    const parentBottom = parRect.bottom;
+
+    const elRect = element.getBoundingClientRect();
+    const elementTop = elRect.top;
+    const elementBottom = elRect.bottom;
+
+    return ((parentTop < elementTop) && (parentBottom > elementBottom));
+  }
+
+  function getLastVisibleMessage() {
+    const parentElement = activeChannel;
+    const childElements = parentElement.getElementsByClassName('message');
+    const lastInView = Array.from(childElements).reverse().find((el) => {
+      return inView(parentElement, el);
+    });
+    return lastInView;
+  }
+
+  function setLastRead() {
+    const channelObject = $channelsList.getChannel(channel);
+    const lastInView = getLastVisibleMessage();
+    if (!channelObject || !lastInView) {
+      return;
+    }
+    const at = DateTime.fromISO(lastInView.dataset.at);
+    // Do it this way, rather than with Math.max tricks, to avoid assignment
+    // when we don't need it, to minimize reactive changes:
+    if (at > channelObject.lastReadAt) {
+      channelObject.lastReadAt = at;
+    }
+  }
+
+  $effect(() => {
+    const _ = $messages.inChannel(channel);
+    setLastRead();
+  });
+
+  function handleKeydown(event) {
+		if (event.key === 'Escape') {
+      setLastRead(); // TODO: pass in "last message DT"?
+    }
+  }
+
+  let lastReadCallback = null;
+  function handleScroll() {
+    clearTimeout(lastReadCallback);  // Fine if lastReadCallback is null still.
+    lastReadCallback = setTimeout(setLastRead, 2 * 1000);
+  }
 </script>
 
-<div class="active-channel">
+<svelte:window onkeydown={handleKeydown} />
+
+<div class="active-channel" on:scroll={handleScroll} bind:this={activeChannel}>
   <ActiveChannel {messageRuns} />
 </div>
 <div class="create-message max-h-full">