Fix sizing bug in `TextEdit::singleline` (#5640)

This PR reverts a change introduced in PR
https://github.com/emilk/egui/pull/3660 that caused a regression with
`TextEdit::singleline`. The original PR attempted to fix an issue with
the cursor in `TextEdit` inside `ScrollArea`, but it did so by adding
unnecessary size allocation to `TextEdit`, which breaks the layout when
`TextEdit::singleline` is used outside of `ScrollArea`.


![Image](https://github.com/user-attachments/assets/78fdf20a-0763-4b5f-b83b-64522f15b35b)

The regression introduced by #3660 is more severe, as it completely
breaks the layout of applications using `TextEdit::singleline`, as shown
in the following issues:

*   Closes https://github.com/emilk/egui/issues/5500
*   Closes https://github.com/emilk/egui/issues/5597

Furthermore, I was unable to reproduce the original bug from PR #3660 in
the current version of egui using the following code:

```rust
impl eframe::App for MyEguiApp {
    fn update(&mut self, ctx: &egui::Context, _: &mut eframe::Frame) {
        ctx.set_debug_on_hover(true);
        egui::CentralPanel::default().show(ctx, |ui| {
            ScrollArea::vertical().max_height(100.0).show(ui, |ui| {
                ui.add(TextEdit::multiline(&mut self.text).hint_text("Enter text here..."))
            });
        });
    }
}
```

This code attempts to recreate the layout shown in the video from PR
#3660, using a `ScrollArea` with limited height and a `TextEdit` inside.
However, the cursor hiding issue was not reproducible.

![Video_2025-01-26_17-54-24](https://github.com/user-attachments/assets/ca4750ea-8af8-4ab5-8c10-bdf73a090362)

Therefore, I believe the code added in PR #3660 is no longer necessary
and only creates more problems.

*   Closes https://github.com/emilk/egui/issues/5500
*   Closes https://github.com/emilk/egui/issues/5597
*   [x] I have followed the instructions in the PR template

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

@@ -730,14 +730,17 @@ impl TextEdit<'_> {
                 }
             }
 
-            // 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(),
-                );
+            if !clip_text {
+                // 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.
+                // Condition `!clip_text` is important to avoid breaking layout for `TextEdit::singleline` (PR #5640)
+                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);

crates/egui_demo_lib/tests/snapshots/demos/Code Example.png

@@ -1,3 +1,3 @@
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-oid sha256:322a50c522ba4ac67206332e1d251e121c8c3d5538ca7961880623b20f4933e5
-size 81732
+oid sha256:0466198f14d15f011e16d16efcc28aeaaf80978ea4e46b5d9a1282304c192c4c
+size 80907