focus-stack: Use IndexSet::shift_remove() for remove

This seems like the correct way to use an `IndexSet`. It shouldn't be
possible to have multiple entries that match, since it's a "set".

We can't define `Borrow<CosmicMapped> for FocusTarget`, so the blanket
impl of `indexmap::Equivalent` won't work, but implementing seems fine.

src/shell/focus/mod.rs

@@ -17,7 +17,7 @@ use smithay::{
         shell::wlr_layer::{KeyboardInteractivity, Layer},
     },
 };
-use std::{borrow::Cow, mem, sync::Mutex};
+use std::{borrow::Cow, hash::Hash, mem, sync::Mutex};
 
 use tracing::{debug, trace};
 
@@ -47,6 +47,18 @@ impl PartialEq<CosmicSurface> for FocusTarget {
     }
 }
 
+impl indexmap::Equivalent<FocusTarget> for CosmicMapped {
+    fn equivalent(&self, key: &FocusTarget) -> bool {
+        key == self
+    }
+}
+
+impl indexmap::Equivalent<FocusTarget> for CosmicSurface {
+    fn equivalent(&self, key: &FocusTarget) -> bool {
+        key == self
+    }
+}
+
 impl From<CosmicMapped> for FocusTarget {
     fn from(value: CosmicMapped) -> Self {
         Self::Window(value)
@@ -119,9 +131,9 @@ impl FocusStackMut<'_> {
 
     pub fn remove<T>(&mut self, target: &T)
     where
-        FocusTarget: PartialEq<T>,
+        T: Hash + indexmap::Equivalent<FocusTarget>,
     {
-        self.0.retain(|w| w != target);
+        self.0.shift_remove(target);
     }
 
     pub fn last(&self) -> Option<&FocusTarget> {