editable_text pointer observers (#23475)

# Objective

Add pointer support for editable text, click to place the cursor and
drag to extend selections.

## Solution

Two new observer systems in the editable_text module, `on_pointer_press`
and `on_pointer_drag`.

Very simple implementation, just calculates the pointer's local position
and queues it with the appropriate `TextEdit` variant.

Author: ickshonpe

_release-content/release-notes/text_input.md

@@ -1,7 +1,7 @@
 ---
 title: "Text input"
 authors: ["@ickshonpe", "@Zeophlite", "@alice-i-cecile"]
-pull_requests: [23282, 23455]
+pull_requests: [23282, 23455, 23475]
 ---
 
 Entering text into an application is a common task, even for games.
@@ -14,6 +14,7 @@ Our initial text entry supports:
 - Press keys on your keyboard, get text (wow!)
 - A cursor that can be moved forward and back in response to the left and right arrow keys
 - Selection rectangles (hold shift)
+- Pointer support, click to place the cursor and drag to extend selection
 - Backspace and Delete
 - Unicode-aware navigation and editing: 1 byte/char != 1 character
 - Bidirectional text support, allowing both left-to-right and right-to-left scripts

crates/bevy_ui_widgets/src/editable_text.rs

@@ -10,9 +10,12 @@ use bevy_app::{App, Plugin};
 use bevy_ecs::prelude::*;
 use bevy_input::keyboard::{Key, KeyCode, KeyboardInput};
 use bevy_input::ButtonInput;
-use bevy_input_focus::FocusedInput;
+use bevy_input_focus::{FocusedInput, InputFocus};
+use bevy_picking::events::{Drag, Pointer, Press};
+use bevy_picking::pointer::PointerButton;
 use bevy_text::{EditableText, TextEdit};
 use bevy_ui::{widget::TextNodeFlags, ContentSize, Node};
+use bevy_ui::{ComputedNode, ComputedUiRenderTargetInfo, UiGlobalTransform, UiScale};
 use smol_str::SmolStr;
 
 /// System that processes keyboard input events into text edit actions for focused [`EditableText`] widgets.
@@ -68,6 +71,89 @@ fn on_focused_keyboard_input(
     trigger.propagate(should_propagate);
 }
 
+/// System that processes pointer press events into text edit actions for [`EditableText`] widgets.
+///
+/// Note that this does not immediately apply the edits; they are queued up in [`EditableText::pending_edits`],
+/// and then applied later by the [`apply_text_edits`](`bevy_text::apply_text_edits`) system.
+fn on_pointer_press(
+    mut press: On<Pointer<Press>>,
+    mut text_input_query: Query<(
+        &mut EditableText,
+        &ComputedNode,
+        &ComputedUiRenderTargetInfo,
+        &UiGlobalTransform,
+    )>,
+    keys: Res<ButtonInput<Key>>,
+    mut input_focus: ResMut<InputFocus>,
+    ui_scale: Res<UiScale>,
+) {
+    if press.button != PointerButton::Primary {
+        return;
+    }
+
+    let Ok((mut editable_text, node, target, transform)) = text_input_query.get_mut(press.entity)
+    else {
+        return;
+    };
+
+    let Some(local_pos) = transform.try_inverse().map(|inverse| {
+        inverse
+            .transform_point2(press.pointer_location.position * target.scale_factor() / ui_scale.0)
+            + 0.5 * node.size()
+    }) else {
+        return;
+    };
+
+    editable_text
+        .pending_edits
+        .push(if keys.pressed(Key::Shift) {
+            TextEdit::ShiftClickExtension
+        } else {
+            TextEdit::MoveToPoint
+        }(local_pos));
+
+    input_focus.set(press.entity);
+
+    press.propagate(false);
+}
+
+/// System that processes pointer drag events into text edit actions for [`EditableText`] widgets.
+///
+/// Note that this does not immediately apply the edits; they are queued up in [`EditableText::pending_edits`],
+/// and then applied later by the [`apply_text_edits`](`bevy_text::apply_text_edits`) system.
+fn on_pointer_drag(
+    mut drag: On<Pointer<Drag>>,
+    mut text_input_query: Query<(
+        &mut EditableText,
+        &ComputedNode,
+        &ComputedUiRenderTargetInfo,
+        &UiGlobalTransform,
+    )>,
+    ui_scale: Res<UiScale>,
+) {
+    if drag.button != PointerButton::Primary {
+        return;
+    }
+
+    let Ok((mut editable_text, node, target, transform)) = text_input_query.get_mut(drag.entity)
+    else {
+        return;
+    };
+
+    let Some(local_pos) = transform.try_inverse().map(|inverse| {
+        inverse
+            .transform_point2(drag.pointer_location.position * target.scale_factor() / ui_scale.0)
+            + 0.5 * node.size()
+    }) else {
+        return;
+    };
+
+    editable_text
+        .pending_edits
+        .push(TextEdit::ExtendSelectionToPoint(local_pos));
+    drag.propagate(false);
+}
+
 /// Enables support for the [`EditableText`] widget.
 ///
 /// Contains the systems and observers necessary to update widget state and handle user input.
@@ -81,7 +167,9 @@ pub struct EditableTextInputPlugin;
 
 impl Plugin for EditableTextInputPlugin {
     fn build(&self, app: &mut App) {
-        app.add_observer(on_focused_keyboard_input);
+        app.add_observer(on_focused_keyboard_input)
+            .add_observer(on_pointer_drag)
+            .add_observer(on_pointer_press);
 
         // These components cannot be registered in `bevy_text` where `EditableText` is defined,
         // because that would create a circular dependency between `bevy_text` and `bevy_ui`.