Lib: Implement (slightly broken) lazy loading of cards

Issues arise when the viewport size changes

src/lib/components/DriverCard.svelte

@@ -43,6 +43,7 @@
 </script>
 
 <LazyCard
+  group="DriverCard"
   imgsrc={driver?.headshot_url ?? headshot_template}
   imgwidth={DRIVER_HEADSHOT_WIDTH}
   imgheight={DRIVER_HEADSHOT_HEIGHT}

src/lib/components/LazyCard.svelte

@@ -1,3 +1,12 @@
+<script lang="ts" module>
+  // The first element of a group of cards (e.g. driver cards or team cards)
+  // will register its height here, once its fully loaded.
+  // This height is then used as the height of following components.
+  // This is necessary, because for lazy loading depending on viewport intersection,
+  // the elements must have their actual height from the beginning.
+  let group_heights: { [key: string]: number } = {};
+</script>
+
 <script lang="ts">
   import type { Snippet } from "svelte";
   import LazyImage from "./LazyImage.svelte";
@@ -23,6 +32,9 @@
 
     /** Enable to give the card the "w-full" class. */
     fullwidth?: boolean;
+
+    /** The group this card belongs to (e.g. "driver" or "race"). All cards that have the same contents (more specifically, height) may be assigned to the same group. */
+    group: string;
   }
 
   let {
@@ -33,6 +45,7 @@
     imgheight,
     imghidden = false,
     fullwidth = false,
+    group,
     ...restProps
   }: CardProps = $props();
 
@@ -41,11 +54,50 @@
   const lazy_visible_handler = () => {
     load = true;
   };
+
+  const set_group_height = (node: HTMLElement) => {
+    if (group_heights[group]) return;
+
+    group_heights[group] = node.getBoundingClientRect().height;
+    // console.log(`Set card group hight: ${group}: ${group_heights[group]}px`);
+  };
 </script>
 
-<div use:lazyload onLazyVisible={lazy_visible_handler}>
-  {#if load}
-    <div class="card overflow-hidden bg-white shadow {fullwidth ? 'w-full' : 'w-auto'}">
+<!-- TODO: This component needs to know its own height, otherwise the intersection observer doesn't work -->
+<!--       (all elements are visible at once, so no lazy loading...) -->
+<div use:lazyload onLazyVisible={lazy_visible_handler} style="width: 100%;">
+  {#if group_heights[group]}
+    <!-- A card has already loaded and determined the height for cards of this group -->
+    <div
+      class="card overflow-hidden bg-white shadow {fullwidth ? 'w-full' : 'w-auto'}"
+      style="height: {group_heights[group]}px;"
+    >
+      <!-- Allow empty strings for images that only appear after user action -->
+      {#if imgsrc !== undefined}
+        <LazyImage
+          id={imgid}
+          src={imgsrc}
+          {imgwidth}
+          {imgheight}
+          alt="Card header"
+          draggable="false"
+          class="select-none shadow"
+          hidden={imghidden}
+        />
+      {/if}
+      <!-- Only lazyload children, as the image is already lazy (also the image fade would break) -->
+      {#if load}
+        <div class="p-2" {...restProps}>
+          {@render children()}
+        </div>
+      {/if}
+    </div>
+  {:else}
+    <!-- This card is the first one to load in the group, so it is not lazy-loaded to determine the height -->
+    <div
+      class="card overflow-hidden bg-white shadow {fullwidth ? 'w-full' : 'w-auto'}"
+      use:set_group_height
+    >
       <!-- Allow empty strings for images that only appear after user action -->
       {#if imgsrc !== undefined}
         <LazyImage

src/lib/components/LazyImage.svelte

@@ -24,7 +24,7 @@
   };
 
   const img_opacity_handler = (node: HTMLElement) => {
-    setTimeout(() => (node.style.opacity = "1"), 10);
+    setTimeout(() => (node.style.opacity = "1"), 20);
   };
 </script>
 
@@ -40,7 +40,7 @@
         src={data}
         use:img_opacity_handler
         class="bg-surface-100 transition-opacity"
-        style="width: 100%; opacity: 0; transition-duration: 250ms;"
+        style="width: 100%; opacity: 0; transition-duration: 500ms;"
         {...restProps}
       />
     {/await}

src/lib/components/RaceCard.svelte

@@ -56,6 +56,7 @@
 </script>
 
 <LazyCard
+  group="RaceCard"
   imgsrc={race?.pictogram_url ?? pictogram_template}
   imgwidth={RACE_PICTOGRAM_WIDTH}
   imgheight={RACE_PICTOGRAM_HEIGHT}

src/lib/components/SubstitutionCard.svelte

@@ -77,6 +77,7 @@
 </script>
 
 <LazyCard
+  group="SubstitutionCard"
   imgsrc={get_by_value(drivers, "id", substitution?.substitute ?? "")?.headshot_url ??
     headshot_template}
   imgwidth={DRIVER_HEADSHOT_WIDTH}

src/lib/components/TeamCard.svelte

@@ -30,6 +30,7 @@
 </script>
 
 <LazyCard
+  group="TeamCard"
   imgsrc={team?.logo_url ?? logo_template}
   imgwidth={TEAM_LOGO_WIDTH}
   imgheight={TEAM_LOGO_HEIGHT}