project panel: Stop flickering border when preview tabs disabled (#20417)

PR #20154 changed the project panel to focus the editor on click in case
preview tabs are disabled.

That lead to a flickering behavior: on mouse-down the border of the
still-selected entry in the project panel would flash, only to disappear
as soon as the entry was opened and editor focused.

This change fixes it by manually keeping track of the mouse-down state,
because we couldn't find a better solution that allows us to simply not
show the border while a "click" is going on.

Release Notes:

- Fixed project panel entries flickering the border when user clicks on
another entry to open it (when preview tabs are disabled.)

Co-authored-by: Piotr <piotr@zed.dev>

crates/project_panel/src/project_panel.rs

@@ -91,6 +91,9 @@ pub struct ProjectPanel {
     horizontal_scrollbar_state: ScrollbarState,
     hide_scrollbar_task: Option<Task<()>>,
     max_width_item_index: Option<usize>,
+    // We keep track of the mouse down state on entries so we don't flash the UI
+    // in case a user clicks to open a file.
+    mouse_down: bool,
 }
 
 #[derive(Clone, Debug)]
@@ -338,6 +341,7 @@ impl ProjectPanel {
                     .parent_view(cx.view()),
                 max_width_item_index: None,
                 scroll_handle,
+                mouse_down: false,
             };
             this.update_visible_entries(None, cx);
 
@@ -2582,12 +2586,21 @@ impl ProjectPanel {
                 this.hover_scroll_task.take();
                 this.drag_onto(selections, entry_id, kind.is_file(), cx);
             }))
+            .on_mouse_down(
+                MouseButton::Left,
+                cx.listener(move |this, _, cx| {
+                    this.mouse_down = true;
+                    cx.propagate();
+                }),
+            )
             .on_click(cx.listener(move |this, event: &gpui::ClickEvent, cx| {
                 if event.down.button == MouseButton::Right || event.down.first_mouse || show_editor
                 {
                     return;
                 }
-
+                if event.down.button == MouseButton::Left {
+                    this.mouse_down = false;
+                }
                 cx.stop_propagation();
 
                 if let Some(selection) = this.selection.filter(|_| event.down.modifiers.shift) {
@@ -2767,7 +2780,7 @@ impl ProjectPanel {
                     .border_color(colors.ghost_element_selected)
             })
             .when(
-                is_active && self.focus_handle.contains_focused(cx),
+                !self.mouse_down && is_active && self.focus_handle.contains_focused(cx),
                 |this| this.border_color(Color::Selected.color(cx)),
             )
     }