Fix cursor clipping in TextEdit inside a ScrollArea (#3660)

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* Closes #1531

### Before
Notice how the cursor hides after third enter and when the line is long.


https://github.com/user-attachments/assets/8e45736e-d6c7-4dc6-94d0-213188c199ff

### After
Cursor is always visible


https://github.com/user-attachments/assets/43200683-3524-471b-990a-eb7b49385fa9


- `ScrollArea` now checks if there's a `scroll_target` in `begin`, if
there is, it saves it because it's not from its children, then restore
it in `end`.
- `TextEdit` now allocates additional space if its galley grows during
the frame. This is needed so that any surrounding `ScrollArea` can bring
the cursor to view, otherwise the cursor lays outside the the
`ScrollArea`'s `content_ui`.

crates/egui/src/containers/scroll_area.rs

@@ -499,6 +499,11 @@ struct Prepared {
 
     scrolling_enabled: bool,
     stick_to_end: Vec2b,
+
+    /// If there was a scroll target before the ScrollArea was added this frame, it's
+    /// not for us to handle so we save it and restore it after this ScrollArea is done.
+    saved_scroll_target: [Option<pass_state::ScrollTarget>; 2],
+
     animated: bool,
 }
 
@@ -693,6 +698,10 @@ impl ScrollArea {
             }
         }
 
+        let saved_scroll_target = content_ui
+            .ctx()
+            .pass_state_mut(|state| std::mem::take(&mut state.scroll_target));
+
         Prepared {
             id,
             state,
@@ -707,6 +716,7 @@ impl ScrollArea {
             viewport,
             scrolling_enabled,
             stick_to_end,
+            saved_scroll_target,
             animated,
         }
     }
@@ -820,6 +830,7 @@ impl Prepared {
             viewport: _,
             scrolling_enabled,
             stick_to_end,
+            saved_scroll_target,
             animated,
         } = self;
 
@@ -853,7 +864,7 @@ impl Prepared {
                     let (start, end) = (range.min, range.max);
                     let clip_start = clip_rect.min[d];
                     let clip_end = clip_rect.max[d];
-                    let mut spacing = ui.spacing().item_spacing[d];
+                    let mut spacing = content_ui.spacing().item_spacing[d];
 
                     let delta_update = if let Some(align) = align {
                         let center_factor = align.to_factor();
@@ -902,6 +913,15 @@ impl Prepared {
             }
         }
 
+        // Restore scroll target meant for ScrollAreas up the stack (if any)
+        ui.ctx().pass_state_mut(|state| {
+            for d in 0..2 {
+                if saved_scroll_target[d].is_some() {
+                    state.scroll_target[d] = saved_scroll_target[d].clone();
+                };
+            }
+        });
+
         let inner_rect = {
             // At this point this is the available size for the inner rect.
             let mut inner_size = inner_rect.size();

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

@@ -1,5 +1,6 @@
 use std::sync::Arc;
 
+use emath::Rect;
 use epaint::text::{cursor::CCursor, Galley, LayoutJob};
 
 use crate::{
@@ -720,17 +721,26 @@ impl<'t> TextEdit<'t> {
                 }
             }
 
+            // Allocate additional space if edits were made this frame that changed the size. This is important so that,
+            // if there's a ScrollArea, it can properly scroll to the cursor.
+            let extra_size = galley.size() - rect.size();
+            if extra_size.x > 0.0 || extra_size.y > 0.0 {
+                ui.allocate_rect(
+                    Rect::from_min_size(outer_rect.max, extra_size),
+                    Sense::hover(),
+                );
+            }
+
             painter.galley(galley_pos, galley.clone(), text_color);
 
             if has_focus {
                 if let Some(cursor_range) = state.cursor.range(&galley) {
                     let primary_cursor_rect =
                         cursor_rect(galley_pos, &galley, &cursor_range.primary, row_height);
 
-                    let is_fully_visible = ui.clip_rect().contains_rect(rect); // TODO(emilk): remove this HACK workaround for https://github.com/emilk/egui/issues/1531
-                    if (response.changed || selection_changed) && !is_fully_visible {
+                    if response.changed || selection_changed {
                         // Scroll to keep primary cursor in view:
-                        ui.scroll_to_rect(primary_cursor_rect, None);
+                        ui.scroll_to_rect(primary_cursor_rect + margin, None);
                     }
 
                     if text.is_mutable() && interactive {