Preferences › Performance › Low-Latency Drawing, on by default: no vsync wait while drawing (#1343)

Testers' traces put the ~50 ms stroke lag in the swap chain's vsync wait. With vsync off it
disappears (X11/NVIDIA; Windows/NVIDIA via the driver), and a shorter queue doesn't help. The
surface now presents with AutoNoVsync by default: wgpu picks Immediate, else Mailbox, else
Fifo by what the driver offers, so no unsupported mode is ever requested. Turning the
preference off brings vsync back (smooth, but late). It applies at the next launch, like the
other graphics settings. Where only Immediate exists the canvas may tear while it changes;
frames render as fast as input arrives during a stroke and stop when idle.

apps/photocraft/src/gpu_startup.rs

@@ -444,6 +444,14 @@ pub fn read_display_server(path: Option<&Path>) -> photocraft_engine::prefs::Lin
 }
 
 /// Read the policy leniently at startup, including old settings without renderingMode.
+/// Preferences › Performance › Low-Latency Drawing (`performance.lowLatencyDrawing`, on unless
+/// set off), read before the window opens: it picks the surface's present mode.
+pub fn read_low_latency(path: Option<&Path>) -> bool {
+    let v: Value = path.and_then(|p| std::fs::read_to_string(p).ok()).and_then(|t| serde_json::from_str(&t).ok()).unwrap_or(Value::Null);
+    let default = photocraft_engine::prefs::Preferences::default().performance.low_latency_drawing;
+    v.get("performance").and_then(|p| p.get("lowLatencyDrawing")).and_then(Value::as_bool).unwrap_or(default)
+}
+
 pub fn read_rendering_prefs(path: Option<&Path>) -> (GpuBackend, RenderingMode) {
     let v: Value = path.and_then(|p| std::fs::read_to_string(p).ok()).and_then(|t| serde_json::from_str(&t).ok()).unwrap_or(Value::Null);
     let perf = v.get("performance");
@@ -463,6 +471,23 @@ mod tests {
         Marker { backend: backend.into(), adapter: "Intel(R) UHD Graphics".into(), adapter_backend: adapter_backend.into(), ..Default::default() }
     }
 
+    #[test]
+    fn low_latency_drawing_is_read_before_the_window_opens_and_defaults_on() {
+        let dir = std::env::temp_dir().join(format!("photocraft-low-latency-{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let file = dir.join("preferences.json");
+        let read = |json: &str| {
+            std::fs::write(&file, json).unwrap();
+            read_low_latency(Some(&file))
+        };
+        assert!(read(r#"{"performance":{"lowLatencyDrawing":true}}"#));
+        assert!(!read(r#"{"performance":{"lowLatencyDrawing":false}}"#));
+        assert!(read(r#"{"performance":{"lowLatencyDrawing":"nope"}}"#), "garbage keeps the default");
+        assert!(read("not json"));
+        assert!(read_low_latency(None), "no settings file: the default");
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
     #[test]
     fn display_server_preference_is_read_before_the_window_opens() {
         use photocraft_engine::prefs::LinuxDisplayServer;

apps/photocraft/src/main.rs

@@ -363,7 +363,12 @@ fn main() -> eframe::Result {
     let gpu_note: std::sync::Arc<std::sync::Mutex<Option<String>>> = Default::default();
     // The adapter's real texture limits (egui asks for 8192 px), so big documents stay on the GPU.
     gpu_startup::configure(&mut options.wgpu_options.wgpu_setup, &plan, os, sentinel.clone(), gpu_note.clone());
-    // Testers' present mode and frame latency overrides (#1343), after the platform default.
+    // Preferences › Performance › Low-Latency Drawing (#1343): without vsync a stroke shows about
+    // 50 ms sooner; wgpu picks Immediate, else Mailbox, else Fifo by what the driver offers.
+    if gpu_startup::read_low_latency(services::prefs_file().as_deref()) {
+        options.wgpu_options.surface.present_mode = eframe::wgpu::PresentMode::AutoNoVsync;
+    }
+    // Testers' present mode and frame latency overrides (#1343), after the preference.
     let present = presentation::Overrides::from_env();
     options.wgpu_options.surface = present.apply(options.wgpu_options.surface);
     let present_desc = presentation::describe(&options.wgpu_options.surface, &present);

crates/engine/src/prefs.rs

@@ -529,6 +529,11 @@ pub struct Performance {
     /// sliders while they drag) render on a reduced copy: fast, but blocky when zoomed in. Off:
     /// they render at full resolution.
     pub low_resolution_previews: bool,
+    /// Low-latency drawing (applies at next launch): frames reach the screen without waiting for
+    /// vsync, so a stroke follows the pen about 50 ms sooner (the swap chain's wait, #1343). Where
+    /// the driver can't replace queued frames (Mailbox), the canvas may tear while it changes.
+    /// Off: vsync, smooth but late.
+    pub low_latency_drawing: bool,
     /// Linux display server (applies at next launch; see [`LinuxDisplayServer`]).
     pub linux_display_server: LinuxDisplayServer,
     /// Memory budget of the layer-effect cache, in MB.
@@ -560,6 +565,7 @@ impl Default for Performance {
             rendering_mode: None,
             gpu_backend: GpuBackend::Auto,
             low_resolution_previews: true,
+            low_latency_drawing: true,
             linux_display_server: LinuxDisplayServer::Auto,
             effect_cache_mb: 768,
             legacy_compositing: false,

crates/ui-egui/src/i18n/cs.tsv

@@ -3783,3 +3783,4 @@ navigator	Zoom	Zvětšení
 # View › Guides › right-click a guide
 	Edit Selected Guides	Upravit vybraná vodítka
 	Clear Selected Guides	Vymazat vybraná vodítka
+	Low-Latency Drawing (vsync off, applies at next launch)	Kreslení s nízkou latencí (bez vertikální synchronizace, platí po dalším spuštění)

crates/ui-egui/src/i18n/de.tsv

@@ -3177,3 +3177,4 @@ curves	Smooth	Glätten
 # View › Guides › right-click a guide
 	Edit Selected Guides	Ausgewählte Hilfslinien bearbeiten
 	Clear Selected Guides	Ausgewählte Hilfslinien löschen
+	Low-Latency Drawing (vsync off, applies at next launch)	Zeichnen mit geringer Latenz (ohne VSync, gilt ab dem nächsten Start)

crates/ui-egui/src/i18n/el.tsv

@@ -3875,3 +3875,4 @@ curves	Smooth	Εξομάλυνση
 # View › Guides › right-click a guide
 	Edit Selected Guides	Επεξεργασία επιλεγμένων οδηγών
 	Clear Selected Guides	Απαλοιφή επιλεγμένων οδηγών
+	Low-Latency Drawing (vsync off, applies at next launch)	Σχεδίαση χαμηλής καθυστέρησης (χωρίς vsync, ισχύει στην επόμενη εκκίνηση)

crates/ui-egui/src/i18n/es.tsv

@@ -4137,3 +4137,4 @@ navigator	Zoom	Zoom
 # View › Guides › right-click a guide
 	Edit Selected Guides	Editar guías seleccionadas
 	Clear Selected Guides	Borrar guías seleccionadas
+	Low-Latency Drawing (vsync off, applies at next launch)	Dibujo de baja latencia (sin sincronización vertical, se aplica al volver a iniciar)

crates/ui-egui/src/i18n/fr.tsv

@@ -3179,3 +3179,4 @@ curves	Smooth	Lisser
 # View › Guides › right-click a guide
 	Edit Selected Guides	Modifier les repères sélectionnés
 	Clear Selected Guides	Effacer les repères sélectionnés
+	Low-Latency Drawing (vsync off, applies at next launch)	Dessin à faible latence (sans synchronisation verticale, au prochain lancement)

crates/ui-egui/src/i18n/hu.tsv

@@ -4094,3 +4094,4 @@ cameraRaw	Yellow	Sárga
 # View › Guides › right-click a guide
 	Edit Selected Guides	Kijelölt segédvonalak szerkesztése
 	Clear Selected Guides	Kijelölt segédvonalak törlése
+	Low-Latency Drawing (vsync off, applies at next launch)	Kis késleltetésű rajzolás (vsync nélkül, a következő indításkor lép életbe)

crates/ui-egui/src/i18n/id.tsv

@@ -3166,3 +3166,4 @@ curves	Smooth	Haluskan
 # View › Guides › right-click a guide
 	Edit Selected Guides	Edit Panduan Terpilih
 	Clear Selected Guides	Hapus Panduan Terpilih
+	Low-Latency Drawing (vsync off, applies at next launch)	Menggambar latensi rendah (tanpa vsync, berlaku saat peluncuran berikutnya)

crates/ui-egui/src/i18n/it.tsv

@@ -4296,3 +4296,4 @@ shape	Appearance	Aspetto
 # View › Guides › right-click a guide
 	Edit Selected Guides	Modifica guide selezionate
 	Clear Selected Guides	Cancella guide selezionate
+	Low-Latency Drawing (vsync off, applies at next launch)	Disegno a bassa latenza (senza vsync, si applica al prossimo avvio)

crates/ui-egui/src/i18n/ja.tsv

@@ -3851,3 +3851,4 @@ curves	Smooth	スムーズ
 # View › Guides › right-click a guide
 	Edit Selected Guides	選択したガイドを編集
 	Clear Selected Guides	選択したガイドを消去
+	Low-Latency Drawing (vsync off, applies at next launch)	低遅延描画（垂直同期オフ、次回起動時に適用）

crates/ui-egui/src/i18n/ko.tsv

@@ -3353,3 +3353,4 @@ curves	Smooth	매끄럽게
 # View › Guides › right-click a guide
 	Edit Selected Guides	선택한 안내선 편집
 	Clear Selected Guides	선택한 안내선 지우기
+	Low-Latency Drawing (vsync off, applies at next launch)	저지연 그리기(수직 동기화 끔, 다음 실행 시 적용)

crates/ui-egui/src/i18n/nl.tsv

@@ -3935,3 +3935,4 @@ curves	Smooth	Vloeiend maken
 # View › Guides › right-click a guide
 	Edit Selected Guides	Geselecteerde hulplijnen bewerken
 	Clear Selected Guides	Geselecteerde hulplijnen wissen
+	Low-Latency Drawing (vsync off, applies at next launch)	Tekenen met lage latentie (zonder vsync, na de volgende start)

crates/ui-egui/src/i18n/pl.tsv

@@ -3773,3 +3773,4 @@ curves	Smooth	Wygładź
 # View › Guides › right-click a guide
 	Edit Selected Guides	Edytuj zaznaczone prowadnice
 	Clear Selected Guides	Usuń zaznaczone prowadnice
+	Low-Latency Drawing (vsync off, applies at next launch)	Rysowanie z małym opóźnieniem (bez synchronizacji pionowej, od następnego uruchomienia)

crates/ui-egui/src/i18n/pt-br.tsv

@@ -3965,3 +3965,4 @@ layerLabel	No Color	Sem cor
 # View › Guides › right-click a guide
 	Edit Selected Guides	Editar guias selecionadas
 	Clear Selected Guides	Limpar guias selecionadas
+	Low-Latency Drawing (vsync off, applies at next launch)	Desenho de baixa latência (sem sincronização vertical, aplicado na próxima inicialização)

crates/ui-egui/src/i18n/ru.tsv

@@ -3144,3 +3144,4 @@ curves	Smooth	Сгладить
 # View › Guides › right-click a guide
 	Edit Selected Guides	Изменить выбранные направляющие
 	Clear Selected Guides	Удалить выбранные направляющие
+	Low-Latency Drawing (vsync off, applies at next launch)	Рисование с малой задержкой (без вертикальной синхронизации, после перезапуска)

crates/ui-egui/src/i18n/sr.tsv

@@ -4248,3 +4248,4 @@ shape	Appearance	Izgled
 # View › Guides › right-click a guide
 	Edit Selected Guides	Uredi izabrane vodilje
 	Clear Selected Guides	Ukloni izabrane vodilje
+	Low-Latency Drawing (vsync off, applies at next launch)	Crtanje sa malim kašnjenjem (bez vsync-a, primenjuje se pri sledećem pokretanju)

crates/ui-egui/src/i18n/tr.tsv

@@ -3905,3 +3905,4 @@ cameraData	Lens	Lens
 # View › Guides › right-click a guide
 	Edit Selected Guides	Seçili Kılavuzları Düzenle
 	Clear Selected Guides	Seçili Kılavuzları Temizle
+	Low-Latency Drawing (vsync off, applies at next launch)	Düşük gecikmeli çizim (dikey eşitleme kapalı, sonraki açılışta geçerli)

crates/ui-egui/src/i18n/uk.tsv

@@ -3874,3 +3874,4 @@ curves	Smooth	Згладити
 # View › Guides › right-click a guide
 	Edit Selected Guides	Редагувати вибрані напрямні
 	Clear Selected Guides	Очистити вибрані напрямні
+	Low-Latency Drawing (vsync off, applies at next launch)	Малювання з малою затримкою (без вертикальної синхронізації, після перезапуску)

crates/ui-egui/src/i18n/vi.tsv

@@ -3661,3 +3661,4 @@ curves	Smooth	Làm mượt
 # View › Guides › right-click a guide
 	Edit Selected Guides	Sửa đường căn đã chọn
 	Clear Selected Guides	Xóa đường căn đã chọn
+	Low-Latency Drawing (vsync off, applies at next launch)	Vẽ độ trễ thấp (tắt vsync, áp dụng ở lần khởi động sau)

crates/ui-egui/src/i18n/zh-hans.tsv

@@ -2790,3 +2790,4 @@ curves	Smooth	平滑
 # View › Guides › right-click a guide
 	Edit Selected Guides	编辑所选参考线
 	Clear Selected Guides	清除所选参考线
+	Low-Latency Drawing (vsync off, applies at next launch)	低延迟绘画（关闭垂直同步，下次启动时生效）

crates/ui-egui/src/i18n/zh-hant.tsv

@@ -2775,3 +2775,4 @@ curves	Smooth	平滑
 # View › Guides › right-click a guide
 	Edit Selected Guides	編輯所選參考線
 	Clear Selected Guides	清除所選參考線
+	Low-Latency Drawing (vsync off, applies at next launch)	低延遲繪圖（關閉垂直同步，下次啟動時生效）

crates/ui-egui/src/prefs_ui.rs

@@ -1077,6 +1077,11 @@ fn humanize(key: &str) -> String {
     if key == "nudgeBundlePauseMs" {
         return "Nudge Bundle Pause (ms)".into();
     }
+    // Performance: presentation without vsync (#1343); like the other graphics settings, it
+    // takes effect when the window next opens.
+    if key == "lowLatencyDrawing" {
+        return "Low-Latency Drawing (vsync off, applies at next launch)".into();
+    }
     let mut s = String::new();
     for (i, ch) in key.chars().enumerate() {
         if i == 0 {

docs/development.md

@@ -120,7 +120,7 @@ By default PhotoCraft's own crates log at `info` and everything else at `warn`.
 | `PHOTOCRAFT_LOCALE` | Override Auto's native UI language for this launch; unsupported tags use English. See [system language detection](localization.md#first-launch-and-system-language). |
 | `PHOTOCRAFT_FX_TRACE=1` | Print the CPU time spent on GPU effect shapes and distance fields per rebuild |
 | `PHOTOCRAFT_STROKE_TRACE=1` (or `=<file.csv>`) | Time every frame of each brush stroke; print a summary per stroke, and with a path also append each frame to that CSV (see [Stroke latency trace](#stroke-latency-trace)) |
-| `PHOTOCRAFT_PRESENT_MODE=novsync` | Present mode for this launch: `fifo`, `autovsync`, or `novsync` (also `immediate`, `mailbox`: no vsync, the first of Immediate, Mailbox, Fifo the driver offers). Default: the platform's. For latency comparisons; shown in the stroke trace |
+| `PHOTOCRAFT_PRESENT_MODE=novsync` | Present mode for this launch: `fifo`, `autovsync`, or `novsync` (also `immediate`, `mailbox`: no vsync, the first of Immediate, Mailbox, Fifo the driver offers). Default: no vsync, or vsync when Preferences › Performance › Low-Latency Drawing is off. For latency comparisons; shown in the stroke trace |
 | `PHOTOCRAFT_FRAME_LATENCY=1` | Frames the swap chain may queue, 1 to 3 (wgpu's GL backend ignores it). For latency comparisons; shown in the stroke trace |
 | `PHOTOCRAFT_THEME_FILE=tokens.json` | **Debug builds only:** live design-token overrides, re-read on change |
 

docs/scorecard.md

@@ -9,7 +9,7 @@ Generated by `cargo xtask scorecard` from `perf/budgets.toml`, `perf/baseline.js
 | [Performance](#performance) | 3 | 22 | 22 | 56 scenarios: 3 within budget, 22 over budget or failing, 9 tracked only, 17 not run, 5 not measurable yet (baseline 2026-10-05) |
 | [File compatibility](#file-compatibility) | 7 | 13 | 7 | io corpus: oracle ≥ 148/170, round trip ≥ 169/170; psd-tools corpus: oracle ≥ 249/309, round trip ≥ 308/309; photoshop oracles: oracle ≥ 185/256, round trip ≥ 258/256 |
 | [Tools](#tools) | 33 | 11 | 0 |  |
-| [Workspace and UI](#workspace-and-ui) | 13 | 6 | 4 | settings that do nothing: 26 of 164 (target 0) |
+| [Workspace and UI](#workspace-and-ui) | 13 | 6 | 4 | settings that do nothing: 26 of 165 (target 0) |
 | [Type](#type) | 4 | 3 | 2 |  |
 | [Automation](#automation) | 5 | 2 | 4 | 631 of 631 menu items live (100.0%) |
 | [Reliability](#reliability) | 5 | 2 | 0 | never-crash lints on 28 of 28 crates; panic_hunt present |
@@ -177,7 +177,7 @@ Corpus floors (`crates/io/tests/corpus.rs`): the enforced minimums; the corpus t
 
 ## Workspace and UI
 
-**Settings that do nothing: 26 of 164** (target 0, #204). Counted by the prefs audit (`xtask/src/scorecard.rs`): a `Preferences` field counts as read when code outside the prefs model (`prefs.rs` except its `impl Session` blocks; tests excluded; the schema-driven dialog never names a field) accesses `section.field`, or `.section` and `.field` in the same file, or calls a section method that reads `self.field`. It is a heuristic lower bound: a generic name can look read when it isn't.
+**Settings that do nothing: 26 of 165** (target 0, #204). Counted by the prefs audit (`xtask/src/scorecard.rs`): a `Preferences` field counts as read when code outside the prefs model (`prefs.rs` except its `impl Session` blocks; tests excluded; the schema-driven dialog never names a field) accesses `section.field`, or `.section` and `.field` in the same file, or calls a section method that reads `self.field`. It is a heuristic lower bound: a generic name can look read when it isn't.
 
 Unread: `general.color_picker`, `general.animated_zoom`, `general.zoom_resizes_windows`, `workspace.auto_show_hidden_panels`, `workspace.enable_floating_document_window_docking`, `file_handling.save_in_background`, `performance.cache_levels`, `performance.legacy_compositing`, `scratch_disks.disks`, `units_and_rulers.column_width`, `units_and_rulers.gutter`, `plug_ins.show_extension_panels`, `plug_ins.allow_scripts_to_connect`, `plug_ins.generator_enabled`, `type_.text_engine`, `type_.recent_fonts`, `enhanced_controls.scrubby_slider_acceleration`, `enhanced_controls.touch_gestures`, `raw_defaults.color_space`, `raw_defaults.bit_depth`, `raw_defaults.resolution`, `raw_defaults.sharpen_for`, `raw_defaults.open_as_smart_object`, `raw_defaults.apply_auto_tone`, `integrations.allow_agent_control`, `integrations.control_port`.