diff --git a/src/Util.js b/src/Util.js
index f073b4306..188c61ef1 100644
--- a/src/Util.js
+++ b/src/Util.js
@@ -29,12 +29,22 @@ export function categoryLabel(category) {
 	return category === '' ? t('notes', 'Uncategorized') : category.replace(/\//g, ' / ')
 }
 
-export function rootCategory(category) {
-	if (!category) {
-		return ''
+/**
+ * Whether a note's category falls within the selected one.
+ *
+ * A null selection matches every note. Otherwise a note matches the selected
+ * category itself and any category nested below it.
+ *
+ * @param {string} noteCategory the note's category
+ * @param {string|null} selectedCategory the selected category, or null for all notes
+ * @return {boolean} whether the note belongs to the selection
+ */
+export function isInCategory(noteCategory, selectedCategory) {
+	if (selectedCategory === null) {
+		return true
 	}
-	const separator = category.indexOf('/')
-	return separator === -1 ? category : category.substring(0, separator)
+	return noteCategory === selectedCategory
+		|| noteCategory.startsWith(selectedCategory + '/')
 }
 
 export function routeIsNewNote($route) {
diff --git a/src/components/NotesView.vue b/src/components/NotesView.vue
index aeae89526..16324a8a4 100644
--- a/src/components/NotesView.vue
+++ b/src/components/NotesView.vue
@@ -93,7 +93,7 @@ import logger from '../Logger.js'
 import { createNote } from '../NotesService.js'
 import store from '../store.js'
 import { fetchNoteTemplates, fetchTemplateContent } from '../TemplateService.js'
-import { categoryLabel, rootCategory } from '../Util.js'
+import { categoryLabel, isInCategory } from '../Util.js'
 
 export default {
 	name: 'NotesView',
@@ -252,7 +252,7 @@ export default {
 			}
 
 			const selectedCategory = store.notes.getSelectedCategory()
-			if (selectedCategory !== null && selectedCategory !== rootCategory(this.note.category)) {
+			if (!isInCategory(this.note.category, selectedCategory)) {
 				this.showNote = false
 			}
 		},
diff --git a/src/tests/Util.spec.js b/src/tests/Util.spec.js
index 8f2dbf502..1fc0fb034 100644
--- a/src/tests/Util.spec.js
+++ b/src/tests/Util.spec.js
@@ -11,9 +11,9 @@ import {
 	getDefaultSampleNote,
 	getDefaultSampleNoteTitle,
 	getDraggedNoteId,
+	isInCategory,
 	isNoteDrag,
 	noteAttributes,
-	rootCategory,
 	routeIsNewNote,
 } from '../Util.js'
 
@@ -81,17 +81,31 @@ describe('categoryLabel', () => {
 	})
 })
 
-describe('rootCategory', () => {
-	it('returns the first category segment', () => {
-		expect(rootCategory('a/b/c')).toBe('a')
+describe('isInCategory', () => {
+	it('matches a note in the selected category itself', () => {
+		expect(isInCategory('Work', 'Work')).toBe(true)
 	})
 
-	it('leaves a plain category alone', () => {
-		expect(rootCategory('Recipes')).toBe('Recipes')
+	it('matches a note in a descendant of the selected category', () => {
+		expect(isInCategory('Work/Projects/2026', 'Work/Projects')).toBe(true)
+	})
+
+	it('matches a note whose nested category is selected exactly', () => {
+		expect(isInCategory('Personal/Work', 'Personal/Work')).toBe(true)
+	})
+
+	it('does not match a sibling category sharing a name prefix', () => {
+		expect(isInCategory('Workshop', 'Work')).toBe(false)
+	})
+
+	it('matches every note when no category is selected', () => {
+		expect(isInCategory('Work/Projects', null)).toBe(true)
+		expect(isInCategory('', null)).toBe(true)
 	})
 
-	it('leaves the empty category empty', () => {
-		expect(rootCategory('')).toBe('')
+	it('matches only uncategorized notes for the uncategorized selection', () => {
+		expect(isInCategory('', '')).toBe(true)
+		expect(isInCategory('Work', '')).toBe(false)
 	})
 })