agent: Make clicking on the backdrop to dismiss message editing more reliable (#31614)

Previously, the click on the backdrop to dismiss the message editing was
unreliable. You would click on it and sometimes it would work and others
it wouldn't. This PR fixes that now.

Release Notes:

- agent: Fixes the previous message dismissal by clicking on the
backdrop

crates/agent/src/active_thread.rs

@@ -1831,6 +1831,7 @@ impl ActiveThread {
 
         let colors = cx.theme().colors();
         let editor_bg_color = colors.editor_background;
+        let panel_bg = colors.panel_background;
 
         let open_as_markdown = IconButton::new(("open-as-markdown", ix), IconName::DocumentText)
             .icon_size(IconSize::XSmall)
@@ -1851,7 +1852,6 @@ impl ActiveThread {
         const RESPONSE_PADDING_X: Pixels = px(19.);
 
         let show_feedback = thread.is_turn_end(ix);
-
         let feedback_container = h_flex()
             .group("feedback_container")
             .mt_1()
@@ -2148,16 +2148,14 @@ impl ActiveThread {
                 message_id > *editing_message_id
             });
 
-        let panel_background = cx.theme().colors().panel_background;
-
         let backdrop = div()
-            .id("backdrop")
-            .stop_mouse_events_except_scroll()
+            .id(("backdrop", ix))
+            .size_full()
             .absolute()
             .inset_0()
-            .size_full()
-            .bg(panel_background)
+            .bg(panel_bg)
             .opacity(0.8)
+            .stop_mouse_events_except_scroll()
             .on_click(cx.listener(Self::handle_cancel_click));
 
         v_flex()