Ensure TextEdit cursor is never out of bounds (upstreamed at: emilk#7077)

When rendering a TextArea we don't know if the saved cursor applies to
the current galley since it's possible the app changed the TextBuffer
(e.g. when submitting a chat input)

So we now detect if the galley changed from the last known one and clamp
the cursor to ensure it's not out of bounds.

This fixes an issue where backspace can suddenly stop working

Repro:
 - Render TextArea with long-ish text (say 20 chars)
 - Without losing focus, clear the text
 - Write something short (say 5 chars)
 - Result: backspace doesn't work because the cursor position is wrong

Author: juancampa

crates/egui/src/text_selection/text_cursor_state.rs

@@ -38,6 +38,14 @@ impl TextCursorState {
     pub fn set_char_range(&mut self, ccursor_range: Option<CCursorRange>) {
         self.ccursor_range = ccursor_range;
     }
+
+    /// Clamp the cursors to be in bounds of the galley.
+    pub fn ensure_in_bounds(&mut self, galley: &Galley) {
+        if let Some(range) = self.ccursor_range.as_mut() {
+            range.primary = galley.clamp_cursor(&range.primary);
+            range.secondary = galley.clamp_cursor(&range.secondary);
+        }
+    }
 }
 
 impl TextCursorState {

crates/egui/src/widgets/text_edit/builder.rs

@@ -553,6 +553,16 @@ impl TextEdit<'_> {
         });
         let mut state = TextEditState::load(ui.ctx(), id).unwrap_or_default();
 
+        // At this point we don't know if the saved cursor still applies to the current galley since
+        // it's possible the app changed the TextBuffer (e.g. when submitting a chat input) so here
+        // we detect if the galley changed from the last known one and clamp the cursor to the new one
+        if let Some(last_galley) = state.last_galley.as_ref() {
+            if last_galley.upgrade().as_ref() != Some(&galley) {
+                state.cursor.ensure_in_bounds(&galley);
+                state.last_galley = Some(Arc::downgrade(&galley));
+            }
+        }
+
         // On touch screens (e.g. mobile in `eframe` web), should
         // dragging select text, or scroll the enclosing [`ScrollArea`] (if any)?
         // Since currently copying selected text in not supported on `eframe` web,
@@ -803,6 +813,7 @@ impl TextEdit<'_> {
             ui.input_mut(|i| i.events.retain(|e| !matches!(e, Event::Ime(_))));
         }
 
+        state.last_galley = Some(Arc::downgrade(&galley));
         state.clone().store(ui.ctx(), id);
 
         if response.changed() {

crates/egui/src/widgets/text_edit/state.rs

@@ -1,4 +1,4 @@
-use std::sync::Arc;
+use std::sync::{Arc, Weak};
 
 use crate::mutex::Mutex;
 
@@ -57,6 +57,10 @@ pub struct TextEditState {
     /// Used to pause the cursor animation when typing.
     #[cfg_attr(feature = "serde", serde(skip))]
     pub(crate) last_interaction_time: f64,
+
+    /// The last galley that was used to render the text. When this changes, we need to ensure the cursor is still in bounds.
+    #[cfg_attr(feature = "serde", serde(skip))]
+    pub(crate) last_galley: Option<Weak<epaint::Galley>>,
 }
 
 impl TextEditState {

crates/epaint/src/text/text_layout_types.rs

@@ -1082,6 +1082,10 @@ impl Galley {
         }
     }
 
+    pub fn clamp_cursor(&self, cursor: &CCursor) -> CCursor {
+        self.cursor_from_layout(self.layout_from_cursor(*cursor))
+    }
+
     pub fn cursor_up_one_row(
         &self,
         cursor: &CCursor,