feat: Handle invisible characters in mail subjects

MailCore/Models/Message.swift

@@ -250,7 +250,7 @@ public final class Message: Object, Decodable, ObjectKeyIdentifiable {
     }
 
     public var formattedSubject: String {
-        return subject ?? MailResourcesStrings.Localizable.noSubjectTitle
+        return SubjectFormatter.cleanNotNullOrEmptySubject.format(subject)
     }
 
     public var displayDate: DisplayDate {

MailCore/Models/Thread.swift

@@ -18,7 +18,6 @@
 
 import Foundation
 import InfomaniakDI
-import MailResources
 import OrderedCollections
 import RealmSwift
 
@@ -125,10 +124,7 @@ public class Thread: Object, Decodable, Identifiable {
     }
 
     public var formattedSubject: String {
-        guard let subject, !subject.isEmpty else {
-            return MailResourcesStrings.Localizable.noSubjectTitle
-        }
-        return subject
+        return SubjectFormatter.cleanNotNullOrEmptySubject.format(subject)
     }
 
     public var shouldPresentAsDraft: Bool {

MailCore/Utils/NotificationsHelper.swift

@@ -325,6 +325,7 @@ public enum NotificationsHelper {
             guard let liveMessage = realm.object(ofType: Message.self, forPrimaryKey: message.uid) else {
                 return
             }
+
             liveMessage.preview = String(preview.prefix(512))
         }
     }

MailCore/Utils/SubjectFormatter.swift

@@ -0,0 +1,78 @@
+/*
+ Infomaniak Mail - iOS App
+ Copyright (C) 2025 Infomaniak Network SA
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import Foundation
+import MailResources
+
+public struct SubjectFormatter: FormatStyle {
+    private func clean(_ value: String) -> String {
+        let normalizedSubject = value.precomposedStringWithCompatibilityMapping
+
+        let cleanedSubject = String(normalizedSubject.unicodeScalars.map { scalar -> Character in
+            if scalar.isIllegalInSubject {
+                return " "
+            } else {
+                return Character(scalar)
+            }
+        })
+
+        return cleanedSubject
+    }
+
+    private func cleanNotNullOrEmpty(_ value: String?) -> String {
+        guard let value, !value.isEmpty else {
+            return MailResourcesStrings.Localizable.noSubjectTitle
+        }
+
+        return clean(value)
+    }
+
+    public func format(_ value: String?) -> String {
+        return cleanNotNullOrEmpty(value)
+    }
+}
+
+public extension FormatStyle where Self == SubjectFormatter {
+    static var cleanNotNullOrEmptySubject: SubjectFormatter {
+        return SubjectFormatter()
+    }
+}
+
+extension UnicodeScalar {
+    var isInvisible: Bool {
+        properties.generalCategory == .format ||
+            properties.generalCategory == .control
+    }
+
+    var isBidiControl: Bool {
+        switch value {
+        case 0x202A ... 0x202E, 0x2066 ... 0x2069:
+            return true
+        default:
+            return false
+        }
+    }
+
+    var isIllegalInSubject: Bool {
+        if isInvisible || properties.isJoinControl || isBidiControl {
+            return true
+        }
+
+        return false
+    }
+}