Fix text selection when there's multiple viewports (#4760)

* Closes #4758

Thanks to @lukexor for finding and diagnosing the problem!

crates/egui/src/context.rs

@@ -774,8 +774,11 @@ impl Context {
     /// ```
     pub fn begin_frame(&self, new_input: RawInput) {
         crate::profile_function!();
-        self.read(|ctx| ctx.plugins.clone()).on_begin_frame(self);
+
         self.write(|ctx| ctx.begin_frame_mut(new_input));
+
+        // Plugs run just after the frame has started:
+        self.read(|ctx| ctx.plugins.clone()).on_begin_frame(self);
     }
 }
 
@@ -1807,6 +1810,7 @@ impl Context {
             crate::gui_zoom::zoom_with_keyboard(self);
         }
 
+        // Plugins run just before the frame ends.
         self.read(|ctx| ctx.plugins.clone()).on_end_frame(self);
 
         #[cfg(debug_assertions)]

crates/egui/src/text_selection/label_text_selection.rs

@@ -154,13 +154,15 @@ impl LabelSelectionState {
     }
 
     pub fn load(ctx: &Context) -> Self {
-        ctx.data(|data| data.get_temp::<Self>(Id::NULL))
+        let id = Id::new(ctx.viewport_id());
+        ctx.data(|data| data.get_temp::<Self>(id))
             .unwrap_or_default()
     }
 
     pub fn store(self, ctx: &Context) {
+        let id = Id::new(ctx.viewport_id());
         ctx.data_mut(|data| {
-            data.insert_temp(Id::NULL, self);
+            data.insert_temp(id, self);
         });
     }