ContentSize replacement fix (bevyengine#9753)

# Objective

If you remove a `ContentSize` component from a Bevy UI entity and then
replace it `ui_layout_system` will remove the measure func from the
internal Taffy layout tree but no new measure func will be generated to
replace it since it's the widget systems that are responsible for
creating their respective measure funcs not `ui_layout_system`. The
widget systems only perform a measure func update on changes to a widget
entity's content. This means that until its content is changed in some
way, no content will be displayed by the node.

### Example

This example spawns a text node which disappears after a few moments
once its `ContentSize` component is replaced.

```rust
use bevy::prelude::*;
use bevy::ui::ContentSize;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, delayed_replacement)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2dBundle::default());
    commands.spawn(
        TextBundle::from_section(
            "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.",
            TextStyle::default(),
        )
    );
}

// Waits a few frames to make sure the font is loaded and the text's glyph layout has been generated.
fn delayed_replacement(mut commands: Commands, mut count: Local<usize>, query: Query<Entity, With<Style>>) {
    *count += 1;
    if *count == 10 {
        for item in query.iter() {
            commands
                .entity(item)
                .remove::<ContentSize>()
                .insert(ContentSize::default());
        }
    }
}
```

## Solution

Perform `ui_layout_system`'s `ContentSize` removal detection and
resolution first, before the measure func updates.
Then in the widget systems, generate a new `Measure` when a
`ContentSize` component is added to a widget entity.

## Changelog

* `measure_text_system`, `update_image_content_size_system` and
`update_atlas_content_size_system` generate a new `Measure` when a
`ContentSize` component is added.

crates/bevy_ui/src/layout/mod.rs

@@ -274,6 +274,11 @@ pub fn ui_layout_system(
         }
     }
 
+    // When a `ContentSize` component is removed from an entity, we need to remove the measure from the corresponding taffy node.
+    for entity in removed_content_sizes.read() {
+        ui_surface.try_remove_measure(entity);
+    }
+
     for (entity, mut content_size) in measure_query.iter_mut() {
         if let Some(measure_func) = content_size.measure_func.take() {
             ui_surface.update_measure(entity, measure_func);
@@ -283,11 +288,6 @@ pub fn ui_layout_system(
     // clean up removed nodes
     ui_surface.remove_entities(removed_nodes.read());
 
-    // When a `ContentSize` component is removed from an entity, we need to remove the measure from the corresponding taffy node.
-    for entity in removed_content_sizes.read() {
-        ui_surface.try_remove_measure(entity);
-    }
-
     // update window children (for now assuming all Nodes live in the primary window)
     ui_surface.set_window_children(primary_window_entity, root_node_query.iter());
 

crates/bevy_ui/src/widget/image.rs

@@ -3,6 +3,7 @@ use crate::{
 };
 use bevy_asset::{Assets, Handle};
 
+use bevy_ecs::change_detection::DetectChanges;
 use bevy_ecs::query::Without;
 use bevy_ecs::{
     prelude::Component,
@@ -96,7 +97,10 @@ pub fn update_image_content_size_system(
                 texture.texture_descriptor.size.height as f32,
             );
             // Update only if size or scale factor has changed to avoid needless layout calculations
-            if size != image_size.size || combined_scale_factor != *previous_combined_scale_factor {
+            if size != image_size.size
+                || combined_scale_factor != *previous_combined_scale_factor
+                || content_size.is_added()
+            {
                 image_size.size = size;
                 content_size.set(ImageMeasure {
                     // multiply the image size by the scale factor to get the physical size
@@ -135,7 +139,10 @@ pub fn update_atlas_content_size_system(
         if let Some(atlas) = atlases.get(atlas) {
             let size = atlas.textures[atlas_image.index].size();
             // Update only if size or scale factor has changed to avoid needless layout calculations
-            if size != image_size.size || combined_scale_factor != *previous_combined_scale_factor {
+            if size != image_size.size
+                || combined_scale_factor != *previous_combined_scale_factor
+                || content_size.is_added()
+            {
                 image_size.size = size;
                 content_size.set(ImageMeasure {
                     // multiply the image size by the scale factor to get the physical size

crates/bevy_ui/src/widget/text.rs

@@ -128,7 +128,7 @@ pub fn measure_text_system(
     if *last_scale_factor == scale_factor {
         // scale factor unchanged, only create new measure funcs for modified text
         for (text, content_size, text_flags) in text_query.iter_mut() {
-            if text.is_changed() || text_flags.needs_new_measure_func {
+            if text.is_changed() || text_flags.needs_new_measure_func || content_size.is_added() {
                 create_text_measure(&fonts, scale_factor, text, content_size, text_flags);
             }
         }