diff --git a/.github/workflows/Test.yml b/.github/workflows/Test.yml
index dae03ab69..8ffbae15d 100644
--- a/.github/workflows/Test.yml
+++ b/.github/workflows/Test.yml
@@ -21,7 +21,7 @@ jobs:
         swiftlint
     - name: Unit Test
       env:
-        DEVELOPER_DIR: /Applications/Xcode_14.3.app
+        DEVELOPER_DIR: /Applications/Xcode_15.0.app
       run: |
         set -o pipefail
         xcodebuild test -project CotEditor.xcodeproj -scheme CotEditor CODE_SIGN_IDENTITY="" CODE_SIGNING_REQUIRED=NO | xcpretty -c
diff --git a/CotEditor.xcodeproj/project.pbxproj b/CotEditor.xcodeproj/project.pbxproj
index 93d4b8ebd..aedffea12 100644
--- a/CotEditor.xcodeproj/project.pbxproj
+++ b/CotEditor.xcodeproj/project.pbxproj
@@ -3,7 +3,7 @@
 	archiveVersion = 1;
 	classes = {
 	};
-	objectVersion = 56;
+	objectVersion = 60;
 	objects = {
 
 /* Begin PBXAggregateTarget section */
@@ -2637,7 +2637,7 @@
 				BuildIndependentTargetsInParallel = YES;
 				CLASSPREFIX = "";
 				LastSwiftUpdateCheck = 1240;
-				LastUpgradeCheck = 1430;
+				LastUpgradeCheck = 1500;
 				ORGANIZATIONNAME = "CotEditor Project";
 				TargetAttributes = {
 					2A3E847D1D07296200070A54 = {
@@ -2683,7 +2683,7 @@
 				};
 			};
 			buildConfigurationList = 8C71D95708640EDF00C9C0BD /* Build configuration list for PBXProject "CotEditor" */;
-			compatibilityVersion = "Xcode 14.0";
+			compatibilityVersion = "Xcode 15.0";
 			developmentRegion = en;
 			hasScannedForEncodings = 1;
 			knownRegions = (
@@ -4343,6 +4343,7 @@
 			isa = XCBuildConfiguration;
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
+				ASSETCATALOG_COMPILER_GENERATE_SWIFT_ASSET_SYMBOL_EXTENSIONS = YES;
 				CLANG_ANALYZER_LOCALIZABILITY_NONLOCALIZED = YES;
 				CLANG_ENABLE_MODULES = YES;
 				CLANG_ENABLE_OBJC_ARC = YES;
@@ -4364,6 +4365,7 @@
 				CLANG_WARN_RANGE_LOOP_ANALYSIS = YES;
 				CLANG_WARN_STRICT_PROTOTYPES = YES;
 				CLANG_WARN_SUSPICIOUS_MOVE = YES;
+				CLANG_WARN_UNGUARDED_AVAILABILITY = YES_AGGRESSIVE;
 				CLANG_WARN_UNREACHABLE_CODE = YES;
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
 				COPY_PHASE_STRIP = NO;
@@ -4371,15 +4373,12 @@
 				DEBUG_INFORMATION_FORMAT = dwarf;
 				ENABLE_STRICT_OBJC_MSGSEND = YES;
 				ENABLE_TESTABILITY = YES;
-				GCC_C_LANGUAGE_STANDARD = gnu11;
+				GCC_C_LANGUAGE_STANDARD = gnu17;
 				GCC_NO_COMMON_BLOCKS = YES;
 				GCC_OPTIMIZATION_LEVEL = 0;
 				GCC_WARN_64_TO_32_BIT_CONVERSION = YES;
 				GCC_WARN_ABOUT_MISSING_NEWLINE = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES_ERROR;
-				GCC_WARN_FOUR_CHARACTER_CONSTANTS = YES;
-				GCC_WARN_INITIALIZER_NOT_FULLY_BRACKETED = YES;
-				GCC_WARN_SHADOW = YES;
 				GCC_WARN_UNDECLARED_SELECTOR = YES;
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
@@ -4390,6 +4389,7 @@
 					"$(inherited)",
 					"@executable_path/../Frameworks",
 				);
+				LOCALIZATION_PREFERS_STRING_CATALOGS = YES;
 				MACOSX_DEPLOYMENT_TARGET = 13.0;
 				MTL_ENABLE_DEBUG_INFO = INCLUDE_SOURCE;
 				MTL_FAST_MATH = YES;
@@ -4399,7 +4399,6 @@
 				SDKROOT = macosx;
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_EMIT_LOC_STRINGS = YES;
-				SWIFT_INSTALL_OBJC_HEADER = NO;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
 				SWIFT_VERSION = 5.0;
 			};
@@ -4409,6 +4408,7 @@
 			isa = XCBuildConfiguration;
 			buildSettings = {
 				ALWAYS_SEARCH_USER_PATHS = NO;
+				ASSETCATALOG_COMPILER_GENERATE_SWIFT_ASSET_SYMBOL_EXTENSIONS = YES;
 				CLANG_ANALYZER_LOCALIZABILITY_NONLOCALIZED = YES;
 				CLANG_ENABLE_MODULES = YES;
 				CLANG_ENABLE_OBJC_ARC = YES;
@@ -4430,6 +4430,7 @@
 				CLANG_WARN_RANGE_LOOP_ANALYSIS = YES;
 				CLANG_WARN_STRICT_PROTOTYPES = YES;
 				CLANG_WARN_SUSPICIOUS_MOVE = YES;
+				CLANG_WARN_UNGUARDED_AVAILABILITY = YES_AGGRESSIVE;
 				CLANG_WARN_UNREACHABLE_CODE = YES;
 				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
 				COPY_PHASE_STRIP = NO;
@@ -4437,14 +4438,11 @@
 				DEBUG_INFORMATION_FORMAT = "dwarf-with-dsym";
 				ENABLE_NS_ASSERTIONS = NO;
 				ENABLE_STRICT_OBJC_MSGSEND = YES;
-				GCC_C_LANGUAGE_STANDARD = gnu11;
+				GCC_C_LANGUAGE_STANDARD = gnu17;
 				GCC_NO_COMMON_BLOCKS = YES;
 				GCC_WARN_64_TO_32_BIT_CONVERSION = YES;
 				GCC_WARN_ABOUT_MISSING_NEWLINE = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES_ERROR;
-				GCC_WARN_FOUR_CHARACTER_CONSTANTS = YES;
-				GCC_WARN_INITIALIZER_NOT_FULLY_BRACKETED = YES;
-				GCC_WARN_SHADOW = YES;
 				GCC_WARN_UNDECLARED_SELECTOR = YES;
 				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
 				GCC_WARN_UNUSED_FUNCTION = YES;
@@ -4455,6 +4453,7 @@
 					"$(inherited)",
 					"@executable_path/../Frameworks",
 				);
+				LOCALIZATION_PREFERS_STRING_CATALOGS = YES;
 				MACOSX_DEPLOYMENT_TARGET = 13.0;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				MTL_FAST_MATH = YES;
@@ -4463,7 +4462,6 @@
 				SDKROOT = macosx;
 				SWIFT_COMPILATION_MODE = wholemodule;
 				SWIFT_EMIT_LOC_STRINGS = YES;
-				SWIFT_INSTALL_OBJC_HEADER = NO;
 				SWIFT_VERSION = 5.0;
 			};
 			name = Release;
diff --git a/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor -Sparkle.xcscheme b/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor -Sparkle.xcscheme
index 1e1a1ada7..981f80ad5 100644
--- a/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor -Sparkle.xcscheme	
+++ b/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor -Sparkle.xcscheme	
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1430"
+   LastUpgradeVersion = "1500"
    version = "2.0">
    <BuildAction
       parallelizeBuildables = "YES"
diff --git a/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor.xcscheme b/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor.xcscheme
index 735bcfeac..5d7060b65 100644
--- a/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor.xcscheme
+++ b/CotEditor.xcodeproj/xcshareddata/xcschemes/CotEditor.xcscheme
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1430"
+   LastUpgradeVersion = "1500"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"
diff --git a/CotEditor.xcodeproj/xcshareddata/xcschemes/UI Tests.xcscheme b/CotEditor.xcodeproj/xcshareddata/xcschemes/UI Tests.xcscheme
index c2e2b3043..9806769e9 100644
--- a/CotEditor.xcodeproj/xcshareddata/xcschemes/UI Tests.xcscheme	
+++ b/CotEditor.xcodeproj/xcshareddata/xcschemes/UI Tests.xcscheme	
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1430"
+   LastUpgradeVersion = "1500"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"
diff --git a/CotEditor.xcodeproj/xcshareddata/xcschemes/Update Help Index.xcscheme b/CotEditor.xcodeproj/xcshareddata/xcschemes/Update Help Index.xcscheme
index 09f3c169c..16e15f9cc 100644
--- a/CotEditor.xcodeproj/xcshareddata/xcschemes/Update Help Index.xcscheme	
+++ b/CotEditor.xcodeproj/xcshareddata/xcschemes/Update Help Index.xcscheme	
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "1430"
+   LastUpgradeVersion = "1500"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"
diff --git a/CotEditor/Sources/WindowContentViewController.swift b/CotEditor/Sources/WindowContentViewController.swift
index 2e8e125e7..6d3331783 100644
--- a/CotEditor/Sources/WindowContentViewController.swift
+++ b/CotEditor/Sources/WindowContentViewController.swift
@@ -127,7 +127,7 @@ final class WindowContentViewController: NSSplitViewController {
     // MARK: Action Messages
     
     /// Toggle visibility of the inspector.
-    @IBAction func toggleInspector(_ sender: Any?) {
+    @IBAction override func toggleInspector(_ sender: Any?) {
         
         NSAnimationContext.current.withAnimation {
             self.isInspectorShown.toggle()
diff --git a/README.md b/README.md
index 67634b5a7..3cb5a4685 100644
--- a/README.md
+++ b/README.md
@@ -22,8 +22,8 @@ CotEditor is a pure document-based Cocoa application written in Swift.
 ### Development Environment
 
 - macOS 13 Ventura
-- Xcode 14.3
-- Swift 5.8
+- Xcode 15
+- Swift 5.9
 - Sandbox enabled