Method push_undo

src/text_input.rs:400–427

Push the current state onto the undo stack before an edit. `kind` controls grouping: consecutive edits of the same kind are merged (only InsertChar, Backspace, and Delete are grouped).

(&mut self, kind: UndoActionKind)

    /// `kind` controls grouping: consecutive edits of the same kind are merged
    /// (only InsertChar, Backspace, and Delete are grouped).
    pub fn push_undo(&mut self, kind: UndoActionKind) {
        // Grouping: if the last undo entry has the same kind and it's a groupable kind,
        // don't push a new entry (the original pre-group state is already saved).
        let should_group = matches!(kind, UndoActionKind::InsertChar | UndoActionKind::Backspace | UndoActionKind::Delete);
        if should_group {
            if let Some(last) = self.undo_stack.last() {
                if last.action_kind == kind {
                    // Same groupable action — skip push, keep the original entry
                    // Clear redo stack since we're making a new edit
                    self.redo_stack.clear();
                    return;
                }
            }
        }

        self.undo_stack.push(UndoEntry {
            text: self.text.clone(),
            cursor_pos: self.cursor_pos,
            selection_anchor: self.selection_anchor,
            action_kind: kind,
        });
        // Limit stack size
        if self.undo_stack.len() > MAX_UNDO_STACK {
            self.undo_stack.remove(0);
        }
        // Any new edit clears the redo stack
        self.redo_stack.clear();
    }

    /// Undo the last edit. Returns true if undo was performed.
    pub fn undo(&mut self) -> bool {