Fix selection clearing in kitty keyboard mode

When Kitty's keyboard protocol is used and Report all keys as escape
codes flag (8) is enabled, modifier key escape codes trigger the usual
"write something to the terminal" code path, which clears the selection
/ scrolls down etc.

This behavior is mostly unexpected, and makes some actions more painful
to perform (for instance copying text becomes harder: hitting CTRL to
begin the CTRL+SHIFT+C sequence clears the selection).

This patch clears the selection only if the key event is not a modifier
key, which aligns with Alacritty's usual behavior.

Fixes #8509.

CHANGELOG.md

@@ -17,6 +17,7 @@ Notable changes to the `alacritty_terminal` crate are documented in its
 ### Fixed
 
 - Crash when OpenGL context resets
+- Modifier keys clearing selection with kitty keyboard protocol enabled
 
 ## 0.15.1
 

alacritty/src/input/keyboard.rs

@@ -77,6 +77,7 @@ impl<T: EventListener, A: ActionContext<T>> Processor<T, A> {
         let mods = if self.alt_send_esc(&key, text) { mods } else { mods & !ModifiersState::ALT };
 
         let build_key_sequence = Self::should_build_sequence(&key, text, mode, mods);
+        let is_modifier_key = Self::is_modifier_key(&key);
 
         let bytes = if build_key_sequence {
             build_sequence(key, mods, mode)
@@ -92,7 +93,10 @@ impl<T: EventListener, A: ActionContext<T>> Processor<T, A> {
 
         // Write only if we have something to write.
         if !bytes.is_empty() {
-            self.ctx.on_terminal_input_start();
+            // Don't clear selection/scroll down when writing escaped modifier keys.
+            if !is_modifier_key {
+                self.ctx.on_terminal_input_start();
+            }
             self.ctx.write_to_pty(bytes);
         }
     }
@@ -125,6 +129,16 @@ impl<T: EventListener, A: ActionContext<T>> Processor<T, A> {
         }
     }
 
+    fn is_modifier_key(key: &KeyEvent) -> bool {
+        matches!(
+            key.logical_key.as_ref(),
+            Key::Named(NamedKey::Shift)
+                | Key::Named(NamedKey::Control)
+                | Key::Named(NamedKey::Alt)
+                | Key::Named(NamedKey::Super)
+        )
+    }
+
     /// Check whether we should try to build escape sequence for the [`KeyEvent`].
     fn should_build_sequence(
         key: &KeyEvent,