Teddy Tennant d802a982ce Don't revert external changes to a focused DragValue (#8403) ...

* Closes <https://github.com/emilk/egui/issues/8339>
* [x] I have followed the instructions in the PR template

## The bug

While a `DragValue` has focus it is rendered as a `TextEdit`, and the
text being
edited is stored in `Memory::data` between frames. That is needed so
that
half-finished input such as `"1."` or `"-"` isn't thrown away just
because it
doesn't parse to the current value.

The stored text was only discarded when the widget *gained* focus or
when the
widget itself changed the value. If something else changed the value
while the
`DragValue` was focused, the stored text was kept, shown to the user,
and
written back to the value when focus was lost — silently undoing the
external
change:

```rust
ui.add(egui::DragValue::new(&mut self.value));
if ui.button("increment").clicked() {
    self.value += 1;
}
```

Click into the `DragValue` so it has focus, then press "increment": the
value
goes up for one frame and then snaps back. `Slider` shows the same
behaviour,
since it uses a `DragValue` for its value field.

## The fix

Store the value the text belongs to next to the text, and discard the
text when
the value no longer matches it. The remembered value is read back from
the
get/set closure *after* the widget has applied its own edits, so a
change the
widget made itself never looks like an external one — this matters for
values
that can't represent what was typed, e.g. `"12.5"` in a
`DragValue<i32>`.

This keeps the reason the text is stored in the first place intact: as
long as
nothing else touches the value, the text the user is typing is preserved
verbatim.

## Tests

Three tests in `crates/egui_kittest/tests/regression_tests.rs`:

* `drag_value_should_not_revert_external_changes_while_focused` — the
actual
  regression. Fails on `main`:

  ```
---- drag_value_should_not_revert_external_changes_while_focused stdout
----
  assertion `left == right` failed
    left: Some("0")
   right: Some("42")
  ```

and, with the display assertion removed so the test reaches the blur, on
the
  value itself:

  ```
  assertion `left == right` failed
    left: 0
   right: 42
  ```

* `drag_value_should_keep_text_while_typing` and
`drag_value_should_keep_text_the_value_cannot_represent` — guards for
the
behaviour the stored text exists for. Both pass on `main` and after the
fix,
  and both fail if the text is re-read from the value too eagerly.

`cargo test -p egui_kittest` and `cargo test -p egui` pass, as do
`cargo fmt --all --check`, `scripts/lint.py` and
`cargo clippy -p egui -p egui_kittest --all-targets --all-features -- -D
warnings`.

## Not changed

`DragValue` still ignores the stored text when <kbd>Escape</kbd> is
pressed, and
`update_while_editing` still decides when typed text is applied —
neither is
touched here.

2026-08-11 14:04:09 +02:00

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