Preferences: Story Editor size stays finite and in range (#821)

Preferences OK stored whatever number the dialog held as the Story
Editor font size. One that is not finite ("nan", "inf", or a number
too large for an f32) was written to ui.json as null; the next launch
could not read the file, started with the default UI settings and
saved them over it on exit. The size is now kept between 8 and 36 pt,
the slider's range, and a value that is not a finite number leaves
the size as it was.

crates/ui-egui/src/dialogs.rs

@@ -2009,7 +2009,10 @@ pub fn confirm(app: &mut DesignApp) -> Result<Value, String> {
                 let words: Vec<String> = d.s("userWords").lines().map(str::to_string).collect();
                 app.run("spelling.setWords", json!({"words": words}))?;
             }
-            app.ui.story_editor_size = d.n("storyEditorSize").unwrap_or(14.0) as f32;
+            // The slider's range. A size that isn't finite is saved as `null`, and ui.json then fails to load.
+            if let Some(size) = d.n("storyEditorSize").filter(|v| v.is_finite()) {
+                app.ui.story_editor_size = size.clamp(8.0, 36.0) as f32;
+            }
             // Highlight options change the screen view only.
             app.canvas.shown = None;
             let view = match d.s("displayQuality").as_str() {
@@ -4095,6 +4098,22 @@ mod tests {
         assert!(app.ui.dialog.as_ref().unwrap().b("absolutePageNumbers"));
     }
 
+    #[test]
+    fn preferences_ok_saves_a_story_editor_size_that_loads_back() {
+        // `ui.dialog.set` takes any value; a size that isn't finite is saved as `null`.
+        let sizes = [(json!("nan"), 20.0), (json!("inf"), 20.0), (json!(null), 20.0), (json!(1e39), 36.0), (json!(3), 8.0), (json!(11), 11.0)];
+        for (size, want) in sizes {
+            let mut app = DesignApp::new(designcraft_engine::Session::new(), crate::Services::default());
+            app.run("file.new", json!({})).unwrap();
+            app.ui.story_editor_size = 20.0;
+            app.run("app.preferences", json!({})).unwrap();
+            app.ui.dialog.as_mut().unwrap().fields.insert("storyEditorSize".into(), size);
+            confirm(&mut app).unwrap();
+            let saved = serde_json::to_vec(&app.ui).unwrap();
+            assert_eq!(crate::UiState::from_preferences_json(&saved).unwrap().story_editor_size, want);
+        }
+    }
+
     #[test]
     fn new_document_vertical_scroll_reaches_last_field() {
         let mut app = DesignApp::new(designcraft_engine::Session::new(), crate::Services::default());