Fix drag_value.edit_string unexpected reset (#2421)

* Fix drag_value.edit_string unexpected reset

  solve issue #2418 #2370

* Also reset drag_value.edit_string on click

* Fix for clippy check

crates/egui/src/widgets/drag_value.rs

@@ -379,6 +379,9 @@ impl<'a> Widget for DragValue<'a> {
         // screen readers.
         ui.memory().interested_in_focus(id);
         let is_kb_editing = ui.memory().has_focus(id);
+        if ui.memory().gained_focus(id) {
+            ui.memory().drag_value.edit_string = None;
+        }
 
         let old_value = get(&mut get_set_value);
         let mut value = old_value;
@@ -476,7 +479,6 @@ impl<'a> Widget for DragValue<'a> {
             ui.memory().drag_value.edit_string = Some(value_text);
             response
         } else {
-            ui.memory().drag_value.edit_string = None;
             let button = Button::new(
                 RichText::new(format!("{}{}{}", prefix, value_text.clone(), suffix)).monospace(),
             )
@@ -497,6 +499,7 @@ impl<'a> Widget for DragValue<'a> {
             }
 
             if response.clicked() {
+                ui.memory().drag_value.edit_string = None;
                 ui.memory().request_focus(id);
             } else if response.dragged() {
                 ui.output().cursor_icon = CursorIcon::ResizeHorizontal;