Calculate ratios and determine compensation strategy: height comp for above/partiality visible, month-scroll comp within a fully visible month.

web/src/lib/components/timeline/Timeline.svelte

@@ -128,20 +128,8 @@
     timelineManager.scrollableElement = scrollableElement;
   });
 
-  const scrollTo = (top: number) => {
-    if (scrollableElement) {
-      scrollableElement.scrollTo({ top });
-    }
-  };
-
-  const scrollTop = (top: number) => {
-    if (scrollableElement) {
-      scrollableElement.scrollTop = top;
-    }
-  };
-
   const scrollToTop = () => {
-    scrollTo(0);
+    timelineManager.scrollTo(0);
   };
 
   const getAssetHeight = (assetId: string, monthGroup: MonthGroup) => monthGroup.findAssetAbsolutePosition(assetId);
@@ -168,8 +156,7 @@
       return true;
     }
 
-    scrollTo(height);
+    timelineManager.scrollTo(height);
-    updateSlidingWindow();
     return true;
   };
 
@@ -179,8 +166,7 @@
       return false;
     }
     const height = getAssetHeight(asset.id, monthGroup);
-    scrollTo(height);
+    timelineManager.scrollTo(height);
-    updateSlidingWindow();
     return true;
   };
 
@@ -235,7 +221,6 @@
 
   const updateIsScrolling = () => (timelineManager.scrolling = true);
   // note: don't throttle, debounch, or otherwise do this function async - it causes flicker
-  const updateSlidingWindow = () => timelineManager.updateSlidingWindow(scrollableElement?.scrollTop || 0);
 
   const topSectionResizeObserver: OnResizeCallback = ({ height }) => (timelineManager.topSectionHeight = height);
 
@@ -267,7 +252,7 @@
     const delta = monthGroup.height * monthGroupScrollPercent;
     const scrollToTop = (topOffset + delta) * maxScrollPercent;
 
-    scrollTop(scrollToTop);
+    timelineManager.scrollTo(scrollToTop);
   };
 
   // note: don't throttle, debounce, or otherwise make this function async - it causes flicker
@@ -279,7 +264,7 @@
       // edge case - scroll limited due to size of content, must adjust - use use the overall percent instead
       const maxScroll = getMaxScroll();
       const offset = maxScroll * overallScrollPercent;
-      scrollTop(offset);
+      timelineManager.scrollTo(offset);
     } else {
       const monthGroup = timelineManager.months.find(
         ({ yearMonth: { year, month } }) => year === scrubberMonth.year && month === scrubberMonth.month,
@@ -421,7 +406,7 @@
     onSelect(asset);
 
     if (singleSelect) {
-      scrollTop(0);
+      timelineManager.scrollTo(0);
       return;
     }
 
@@ -587,9 +572,9 @@
   style:margin-right={(usingMobileDevice ? 0 : scrubberWidth) + 'px'}
   tabindex="-1"
   bind:clientHeight={timelineManager.viewportHeight}
-  bind:clientWidth={null, (v: number) => ((timelineManager.viewportWidth = v), updateSlidingWindow())}
+  bind:clientWidth={timelineManager.viewportWidth}
   bind:this={scrollableElement}
-  onscroll={() => (handleTimelineScroll(), updateSlidingWindow(), updateIsScrolling())}
+  onscroll={() => (handleTimelineScroll(), timelineManager.updateSlidingWindow(), updateIsScrolling())}
 >
   <section
     bind:this={timelineElement}

web/src/lib/components/timeline/TimelineDateGroup.svelte

@@ -14,7 +14,7 @@
 
   import { fromTimelinePlainDate, getDateLocaleString } from '$lib/utils/timeline-util';
   import { Icon } from '@immich/ui';
-  import type { Snippet } from 'svelte';
+  import { type Snippet } from 'svelte';
   import { flip } from 'svelte/animate';
   import { scale } from 'svelte/transition';
 

web/src/lib/managers/timeline-manager/internal/load-support.svelte.ts

@@ -4,7 +4,6 @@ import { getTimeBucket } from '@immich/sdk';
 import type { MonthGroup } from '../month-group.svelte';
 import type { TimelineManager } from '../timeline-manager.svelte';
 import type { TimelineManagerOptions } from '../types';
-import { layoutMonthGroup } from './layout-support.svelte';
 
 export async function loadFromTimeBuckets(
   timelineManager: TimelineManager,
@@ -55,6 +54,4 @@ export async function loadFromTimeBuckets(
       )}`,
     );
   }
-
-  layoutMonthGroup(timelineManager, monthGroup);
 }

web/src/lib/managers/timeline-manager/month-group.svelte.ts

@@ -263,14 +263,19 @@ export class MonthGroup {
         monthGroup.#top = newTop;
       }
     }
-    const intersectingMonth = timelineManager.topIntersectingMonthGroup;
+    if (!timelineManager.viewportTopMonthIntersection) {
-    const currentIndex = intersectingMonth ? timelineManager.months.indexOf(intersectingMonth) : -1;
-    if (!intersectingMonth || currentIndex <= 0) {
       return;
     }
-
+    const { month, monthBottomViewportRatio, viewportTopRatioInMonth } = timelineManager.viewportTopMonthIntersection;
-    if (index <= currentIndex) {
+    const currentIndex = month ? timelineManager.months.indexOf(month) : -1;
+    if (!month || currentIndex <= 0 || index > currentIndex) {
+      return;
+    }
+    if (index < currentIndex || monthBottomViewportRatio < 1) {
       timelineManager.scrollBy(heightDelta);
+    } else if (index === currentIndex) {
+      const scrollTo = this.top + height * viewportTopRatioInMonth;
+      timelineManager.scrollTo(scrollTo);
     }
   }
 

web/src/lib/managers/timeline-manager/timeline-manager.svelte.spec.ts

@@ -4,6 +4,7 @@ import { AbortError } from '$lib/utils';
 import { fromISODateTimeUTCToObject } from '$lib/utils/timeline-util';
 import { type AssetResponseDto, type TimeBucketAssetResponseDto } from '@immich/sdk';
 import { timelineAssetFactory, toResponseDto } from '@test-data/factories/asset-factory';
+import { tick } from 'svelte';
 import { TimelineManager } from './timeline-manager.svelte';
 import type { TimelineAsset } from './types';
 
@@ -64,6 +65,7 @@ describe('TimelineManager', () => {
 
       sdkMock.getTimeBucket.mockImplementation(({ timeBucket }) => Promise.resolve(bucketAssetsResponse[timeBucket]));
       await timelineManager.updateViewport({ width: 1588, height: 1000 });
+      await tick();
     });
 
     it('should load months in viewport', () => {

web/src/lib/managers/timeline-manager/timeline-manager.svelte.ts

@@ -5,7 +5,7 @@ import { authManager } from '$lib/managers/auth-manager.svelte';
 import { CancellableTask } from '$lib/utils/cancellable-task';
 import { toTimelineAsset, type TimelineDateTime, type TimelineYearMonth } from '$lib/utils/timeline-util';
 
-import { debounce, isEqual } from 'lodash-es';
+import { clamp, debounce, isEqual } from 'lodash-es';
 import { SvelteDate, SvelteMap, SvelteSet } from 'svelte/reactivity';
 
 import { updateIntersectionMonthGroup } from '$lib/managers/timeline-manager/internal/intersection-support.svelte';
@@ -37,6 +37,13 @@ import type {
   Viewport,
 } from './types';
 
+type ViewportTopMonthIntersection = {
+  month: MonthGroup | undefined;
+  // Where viewport top intersects month (0 = month top, 1 = month bottom)
+  viewportTopRatioInMonth: number;
+  // Where month bottom is in viewport (0 = viewport top, 1 = viewport bottom)
+  monthBottomViewportRatio: number;
+};
 export class TimelineManager {
   isInitialized = $state(false);
   months: MonthGroup[] = $state([]);
@@ -48,7 +55,8 @@ export class TimelineManager {
 
   scrubberMonths: ScrubberMonth[] = $state([]);
   scrubberTimelineHeight: number = $state(0);
-  topIntersectingMonthGroup: MonthGroup | undefined = $state();
+
+  viewportTopMonthIntersection: ViewportTopMonthIntersection | undefined;
 
   visibleWindow = $derived.by(() => ({
     top: this.#scrollTop,
@@ -87,7 +95,7 @@ export class TimelineManager {
   #resetScrolling = debounce(() => (this.#scrolling = false), 1000);
   #resetSuspendTransitions = debounce(() => (this.suspendTransitions = false), 1000);
   #updatingIntersections = false;
-  #element: HTMLElement | undefined;
+  #scrollableElement: HTMLElement | undefined = $state();
 
   constructor() {}
 
@@ -102,15 +110,17 @@ export class TimelineManager {
   }
 
   set scrollableElement(element: HTMLElement | undefined) {
-    this.#element = element;
+    this.#scrollableElement = element;
   }
 
   scrollTo(top: number) {
-    this.#element?.scrollTo({ top });
+    this.#scrollableElement?.scrollTo({ top });
+    this.updateSlidingWindow();
   }
 
   scrollBy(y: number) {
-    this.#element?.scrollBy(0, y);
+    this.#scrollableElement?.scrollBy(0, y);
+    this.updateSlidingWindow();
   }
 
   #setHeaderHeight(value: number) {
@@ -176,7 +186,8 @@ export class TimelineManager {
     const changed = value !== this.#viewportWidth;
     this.#viewportWidth = value;
     this.suspendTransitions = true;
-    void this.#updateViewportGeometry(changed);
+    this.#updateViewportGeometry(changed);
+    this.updateSlidingWindow();
   }
 
   get viewportWidth() {
@@ -238,13 +249,31 @@ export class TimelineManager {
     this.#websocketSupport = undefined;
   }
 
-  updateSlidingWindow(scrollTop: number) {
+  updateSlidingWindow() {
+    const scrollTop = this.#scrollableElement?.scrollTop ?? 0;
     if (this.#scrollTop !== scrollTop) {
       this.#scrollTop = scrollTop;
       this.updateIntersections();
     }
   }
 
+  #calculateMonthBottomViewportRatio(month: MonthGroup | undefined) {
+    if (!month) {
+      return 0;
+    }
+    const windowHeight = this.visibleWindow.bottom - this.visibleWindow.top;
+    const bottomOfMonth = month.top + month.height;
+    const bottomOfMonthInViewport = bottomOfMonth - this.visibleWindow.top;
+    return clamp(bottomOfMonthInViewport / windowHeight, 0, 1);
+  }
+
+  #calculateVewportTopRatioInMonth(month: MonthGroup | undefined) {
+    if (!month) {
+      return 0;
+    }
+    return clamp((this.visibleWindow.top - month.top) / month.height, 0, 1);
+  }
+
   updateIntersections() {
     if (this.#updatingIntersections || !this.isInitialized || this.visibleWindow.bottom === this.visibleWindow.top) {
       return;
@@ -255,7 +284,16 @@ export class TimelineManager {
       updateIntersectionMonthGroup(this, month);
     }
 
-    this.topIntersectingMonthGroup = this.months.find((month) => month.actuallyIntersecting);
+    const month = this.months.find((month) => month.actuallyIntersecting);
+    const viewportTopRatioInMonth = this.#calculateVewportTopRatioInMonth(month);
+    const monthBottomViewportRatio = this.#calculateMonthBottomViewportRatio(month);
+
+    this.viewportTopMonthIntersection = {
+      month,
+      monthBottomViewportRatio,
+      viewportTopRatioInMonth,
+    };
+
     this.#updatingIntersections = false;
   }
 
@@ -388,7 +426,8 @@ export class TimelineManager {
       await loadFromTimeBuckets(this, monthGroup, this.#options, signal);
     }, cancelable);
     if (executionStatus === 'LOADED') {
-      updateIntersectionMonthGroup(this, monthGroup);
+      updateGeometry(this, monthGroup, { invalidateHeight: false });
+      this.updateIntersections();
     }
   }