Avoid clearing chat overlay textbox when pressing "back" key binding

Generally this is expected behaviour for usages of focused text boxes
(ie. to clear search content), but not so much here.

Addresses https://github.com/ppy/osu/discussions/19403#discussioncomment-3230395.

osu.Game/Graphics/UserInterface/FocusedTextBox.cs

@@ -21,6 +21,11 @@ public class FocusedTextBox : OsuTextBox, IKeyBindingHandler<GlobalAction>
 
         private bool allowImmediateFocus => host?.OnScreenKeyboardOverlapsGameWindow != true;
 
+        /// <summary>
+        /// Whether the content of the text box should be cleared on the first "back" key press.
+        /// </summary>
+        protected virtual bool ClearTextOnBackKey => true;
+
         public void TakeFocus()
         {
             if (!allowImmediateFocus)
@@ -78,7 +83,7 @@ public virtual bool OnPressed(KeyBindingPressEvent<GlobalAction> e)
 
             if (!HasFocus) return false;
 
-            if (e.Action == GlobalAction.Back)
+            if (ClearTextOnBackKey && e.Action == GlobalAction.Back)
             {
                 if (Text.Length > 0)
                 {

osu.Game/Overlays/Chat/ChatTextBox.cs

@@ -13,6 +13,8 @@ public class ChatTextBox : FocusedTextBox
 
         public override bool HandleLeftRightArrows => !ShowSearch.Value;
 
+        protected override bool ClearTextOnBackKey => false;
+
         protected override void LoadComplete()
         {
             base.LoadComplete();