Add `Ui::is_sizing_pass` for better size estimation of `Area`s, and menus in particular (#4557)

* Part of https://github.com/emilk/egui/issues/4535
* Closes https://github.com/emilk/egui/issues/3974

This adds a special `sizing_pass` mode to `Ui`, in which we have no
centered or justified layouts, and everything is hidden. This is used by
`Area` to use the first frame to measure the size of its contents so
that it can then set the perfectly correct size the subsequent frames.

For menus, where buttons are justified (span the full width), this
finally the problem of auto-sizing. Before you would have to pick a
width manually, and all buttons would expand to that width. If it was
too wide, it looked weird. If it was too narrow, text would wrap. Now
all menus are exactly the width they need to be. By default menus will
wrap at `Spacing::menu_width`.

This affects all situations when you have something that should be as
small as possible, but still span the full width/height of the parent.
For instance: the `egui::Separator` widget now checks the
`ui.is_sizing_pass` flag before deciding on a size. In the sizing pass a
horizontal separator is always 0 wide, and only in subsequent passes
will it span the full width.

Cargo.lock

@@ -3628,6 +3628,14 @@ dependencies = [
  "env_logger",
 ]
 
+[[package]]
+name = "test_size_pass"
+version = "0.1.0"
+dependencies = [
+ "eframe",
+ "env_logger",
+]
+
 [[package]]
 name = "test_viewports"
 version = "0.1.0"

crates/egui/src/containers/area.rs

@@ -69,6 +69,7 @@ pub struct Area {
     constrain_rect: Option<Rect>,
     order: Order,
     default_pos: Option<Pos2>,
+    default_size: Vec2,
     pivot: Align2,
     anchor: Option<(Align2, Vec2)>,
     new_pos: Option<Pos2>,
@@ -87,6 +88,7 @@ impl Area {
             enabled: true,
             order: Order::Middle,
             default_pos: None,
+            default_size: Vec2::NAN,
             new_pos: None,
             pivot: Align2::LEFT_TOP,
             anchor: None,
@@ -163,6 +165,35 @@ impl Area {
         self
     }
 
+    /// The size used for the [`Ui::max_rect`] the first frame.
+    ///
+    /// Text will wrap at this width, and images that expand to fill the available space
+    /// will expand to this size.
+    ///
+    /// If the contents are smaller than this size, the area will shrink to fit the contents.
+    /// If the contents overflow, the area will grow.
+    ///
+    /// If not set, [`style::Spacing::default_area_size`] will be used.
+    #[inline]
+    pub fn default_size(mut self, default_size: impl Into<Vec2>) -> Self {
+        self.default_size = default_size.into();
+        self
+    }
+
+    /// See [`Self::default_size`].
+    #[inline]
+    pub fn default_width(mut self, default_width: f32) -> Self {
+        self.default_size.x = default_width;
+        self
+    }
+
+    /// See [`Self::default_size`].
+    #[inline]
+    pub fn default_height(mut self, default_height: f32) -> Self {
+        self.default_size.y = default_height;
+        self
+    }
+
     /// Positions the window and prevents it from being moved
     #[inline]
     pub fn fixed_pos(mut self, fixed_pos: impl Into<Pos2>) -> Self {
@@ -247,7 +278,7 @@ pub(crate) struct Prepared {
     /// This is so that we use the first frame to calculate the window size,
     /// and then can correctly position the window and its contents the next frame,
     /// without having one frame where the window is wrongly positioned or sized.
-    temporarily_invisible: bool,
+    sizing_pass: bool,
 }
 
 impl Area {
@@ -272,6 +303,7 @@ impl Area {
             interactable,
             enabled,
             default_pos,
+            default_size,
             new_pos,
             pivot,
             anchor,
@@ -292,11 +324,29 @@ impl Area {
         if is_new {
             ctx.request_repaint(); // if we don't know the previous size we are likely drawing the area in the wrong place
         }
-        let mut state = state.unwrap_or_else(|| State {
+        let mut state = state.unwrap_or_else(|| {
+            // during the sizing pass we will use this as the max size
+            let mut size = default_size;
+
+            let default_area_size = ctx.style().spacing.default_area_size;
+            if size.x.is_nan() {
+                size.x = default_area_size.x;
+            }
+            if size.y.is_nan() {
+                size.y = default_area_size.y;
+            }
+
+            if constrain {
+                let constrain_rect = constrain_rect.unwrap_or_else(|| ctx.screen_rect());
+                size = size.at_most(constrain_rect.size());
+            }
+
+            State {
                 pivot_pos: default_pos.unwrap_or_else(|| automatic_area_position(ctx)),
                 pivot,
-            size: Vec2::ZERO,
+                size,
                 interactable,
+            }
         });
         state.pivot_pos = new_pos.unwrap_or(state.pivot_pos);
         state.interactable = interactable;
@@ -365,7 +415,7 @@ impl Area {
             enabled,
             constrain,
             constrain_rect,
-            temporarily_invisible: is_new,
+            sizing_pass: is_new,
         }
     }
 
@@ -431,12 +481,7 @@ impl Prepared {
             }
         };
 
-        let max_rect = Rect::from_min_max(
-            self.state.left_top_pos(),
-            constrain_rect
-                .max
-                .at_least(self.state.left_top_pos() + Vec2::splat(32.0)),
-        );
+        let max_rect = Rect::from_min_size(self.state.left_top_pos(), self.state.size);
 
         let clip_rect = constrain_rect; // Don't paint outside our bounds
 
@@ -448,7 +493,9 @@ impl Prepared {
             clip_rect,
         );
         ui.set_enabled(self.enabled);
-        ui.set_visible(!self.temporarily_invisible);
+        if self.sizing_pass {
+            ui.set_sizing_pass();
+        }
         ui
     }
 
@@ -461,11 +508,20 @@ impl Prepared {
             enabled: _,
             constrain: _,
             constrain_rect: _,
-            temporarily_invisible: _,
+            sizing_pass,
         } = self;
 
         state.size = content_ui.min_size();
 
+        if sizing_pass {
+            // If during the sizing pass we measure our width to `123.45` and
+            // then try to wrap to exactly that next frame,
+            // we may accidentally wrap the last letter of some text.
+            // We only do this after the initial sizing pass though;
+            // otherwise we could end up with for-ever expanding areas.
+            state.size = state.size.ceil();
+        }
+
         ctx.memory_mut(|m| m.areas_mut().set_state(layer_id, state));
 
         move_response

crates/egui/src/containers/frame.rs

@@ -266,7 +266,8 @@ impl Frame {
         self.show_dyn(ui, Box::new(add_contents))
     }
 
-    fn show_dyn<'c, R>(
+    /// Show using dynamic dispatch.
+    pub fn show_dyn<'c, R>(
         self,
         ui: &mut Ui,
         add_contents: Box<dyn FnOnce(&mut Ui) -> R + 'c>,

crates/egui/src/containers/popup.rs

@@ -260,15 +260,11 @@ fn show_tooltip_area_dyn<'c, R>(
     Area::new(area_id)
         .order(Order::Tooltip)
         .fixed_pos(window_pos)
+        .default_width(ctx.style().spacing.tooltip_width)
         .constrain_to(ctx.screen_rect())
         .interactable(false)
         .show(ctx, |ui| {
-            Frame::popup(&ctx.style())
-                .show(ui, |ui| {
-                    ui.set_max_width(ui.spacing().tooltip_width);
-                    add_contents(ui)
-                })
-                .inner
+            Frame::popup(&ctx.style()).show_dyn(ui, add_contents).inner
         })
 }
 

crates/egui/src/menu.rs

@@ -133,10 +133,10 @@ pub(crate) fn submenu_button<R>(
 }
 
 /// wrapper for the contents of every menu.
-pub(crate) fn menu_ui<'c, R>(
+fn menu_popup<'c, R>(
     ctx: &Context,
-    menu_id: Id,
     menu_state_arc: &Arc<RwLock<MenuState>>,
+    menu_id: Id,
     add_contents: impl FnOnce(&mut Ui) -> R + 'c,
 ) -> InnerResponse<R> {
     let pos = {
@@ -150,6 +150,7 @@ pub(crate) fn menu_ui<'c, R>(
         .fixed_pos(pos)
         .constrain_to(ctx.screen_rect())
         .interactable(true)
+        .default_width(ctx.style().spacing.menu_width)
         .sense(Sense::hover());
 
     let area_response = area.show(ctx, |ui| {
@@ -157,7 +158,6 @@ pub(crate) fn menu_ui<'c, R>(
 
         Frame::menu(ui.style())
             .show(ui, |ui| {
-                ui.set_max_width(ui.spacing().menu_width);
                 ui.set_menu_state(Some(menu_state_arc.clone()));
                 ui.with_layout(Layout::top_down_justified(Align::LEFT), add_contents)
                     .inner
@@ -306,8 +306,7 @@ impl MenuRoot {
         add_contents: impl FnOnce(&mut Ui) -> R,
     ) -> (MenuResponse, Option<InnerResponse<R>>) {
         if self.id == button.id {
-            let inner_response =
-                MenuState::show(&button.ctx, &self.menu_state, self.id, add_contents);
+            let inner_response = menu_popup(&button.ctx, &self.menu_state, self.id, add_contents);
             let menu_state = self.menu_state.read();
 
             if menu_state.response.is_close() {
@@ -593,15 +592,6 @@ impl MenuState {
         self.response = MenuResponse::Close;
     }
 
-    pub fn show<R>(
-        ctx: &Context,
-        menu_state: &Arc<RwLock<Self>>,
-        id: Id,
-        add_contents: impl FnOnce(&mut Ui) -> R,
-    ) -> InnerResponse<R> {
-        crate::menu::menu_ui(ctx, id, menu_state, add_contents)
-    }
-
     fn show_submenu<R>(
         &mut self,
         ctx: &Context,
@@ -609,7 +599,7 @@ impl MenuState {
         add_contents: impl FnOnce(&mut Ui) -> R,
     ) -> Option<R> {
         let (sub_response, response) = self.submenu(id).map(|sub| {
-            let inner_response = Self::show(ctx, sub, id, add_contents);
+            let inner_response = menu_popup(ctx, sub, id, add_contents);
             (sub.read().response, inner_response.inner)
         })?;
         self.cascade_close_response(sub_response);

crates/egui/src/style.rs

@@ -316,10 +316,21 @@ pub struct Spacing {
     /// This is the spacing between the icon and the text
     pub icon_spacing: f32,
 
+    /// The size used for the [`Ui::max_rect`] the first frame.
+    ///
+    /// Text will wrap at this width, and images that expand to fill the available space
+    /// will expand to this size.
+    ///
+    /// If the contents are smaller than this size, the area will shrink to fit the contents.
+    /// If the contents overflow, the area will grow.
+    pub default_area_size: Vec2,
+
     /// Width of a tooltip (`on_hover_ui`, `on_hover_text` etc).
     pub tooltip_width: f32,
 
-    /// The default width of a menu.
+    /// The default wrapping width of a menu.
+    ///
+    /// Items longer than this will wrap to a new line.
     pub menu_width: f32,
 
     /// Horizontal distance between a menu and a submenu.
@@ -1073,8 +1084,9 @@ impl Default for Spacing {
             icon_width: 14.0,
             icon_width_inner: 8.0,
             icon_spacing: 4.0,
+            default_area_size: vec2(600.0, 400.0),
             tooltip_width: 600.0,
-            menu_width: 150.0,
+            menu_width: 400.0,
             menu_spacing: 2.0,
             combo_height: 200.0,
             scroll: Default::default(),
@@ -1459,6 +1471,7 @@ impl Spacing {
             icon_width,
             icon_width_inner,
             icon_spacing,
+            default_area_size,
             tooltip_width,
             menu_width,
             menu_spacing,
@@ -1509,6 +1522,10 @@ impl Spacing {
                 ui.add(DragValue::new(combo_width).clamp_range(0.0..=1000.0));
                 ui.end_row();
 
+                ui.label("Default area size");
+                ui.add(two_drag_values(default_area_size, 0.0..=1000.0));
+                ui.end_row();
+
                 ui.label("TextEdit width");
                 ui.add(DragValue::new(text_edit_width).clamp_range(0.0..=1000.0));
                 ui.end_row();

crates/egui/src/ui.rs

@@ -61,6 +61,10 @@ pub struct Ui {
     /// and all widgets will assume a gray style.
     enabled: bool,
 
+    /// Set to true in special cases where we do one frame
+    /// where we size up the contents of the Ui, without actually showing it.
+    sizing_pass: bool,
+
     /// Indicates whether this Ui belongs to a Menu.
     menu_state: Option<Arc<RwLock<MenuState>>>,
 }
@@ -82,6 +86,7 @@ impl Ui {
             style,
             placer: Placer::new(max_rect, Layout::default()),
             enabled: true,
+            sizing_pass: false,
             menu_state: None,
         };
 
@@ -108,9 +113,18 @@ impl Ui {
     pub fn child_ui_with_id_source(
         &mut self,
         max_rect: Rect,
-        layout: Layout,
+        mut layout: Layout,
         id_source: impl Hash,
     ) -> Self {
+        if self.sizing_pass {
+            // During the sizing pass we want widgets to use up as little space as possible,
+            // so that we measure the only the space we _need_.
+            layout.cross_justify = false;
+            if layout.cross_align == Align::Center {
+                layout.cross_align = Align::Min;
+            }
+        }
+
         debug_assert!(!max_rect.any_nan());
         let next_auto_id_source = Id::new(self.next_auto_id_source).with("child").value();
         self.next_auto_id_source = self.next_auto_id_source.wrapping_add(1);
@@ -121,6 +135,7 @@ impl Ui {
             style: self.style.clone(),
             placer: Placer::new(max_rect, layout),
             enabled: self.enabled,
+            sizing_pass: self.sizing_pass,
             menu_state: self.menu_state.clone(),
         };
 
@@ -140,6 +155,26 @@ impl Ui {
 
     // -------------------------------------------------
 
+    /// Set to true in special cases where we do one frame
+    /// where we size up the contents of the Ui, without actually showing it.
+    ///
+    /// This will also turn the Ui invisible.
+    /// Should be called right after [`Self::new`], if at all.
+    #[inline]
+    pub fn set_sizing_pass(&mut self) {
+        self.sizing_pass = true;
+        self.set_visible(false);
+    }
+
+    /// Set to true in special cases where we do one frame
+    /// where we size up the contents of the Ui, without actually showing it.
+    #[inline]
+    pub fn is_sizing_pass(&self) -> bool {
+        self.sizing_pass
+    }
+
+    // -------------------------------------------------
+
     /// A unique identity of this [`Ui`].
     #[inline]
     pub fn id(&self) -> Id {

crates/egui/src/widgets/separator.rs

@@ -96,7 +96,11 @@ impl Widget for Separator {
         let is_horizontal_line = is_horizontal_line
             .unwrap_or_else(|| ui.is_grid() || !ui.layout().main_dir().is_horizontal());
 
-        let available_space = ui.available_size_before_wrap();
+        let available_space = if ui.is_sizing_pass() {
+            Vec2::ZERO
+        } else {
+            ui.available_size_before_wrap()
+        };
 
         let size = if is_horizontal_line {
             vec2(available_space.x, spacing)

crates/egui_demo_lib/src/demo/context_menu.rs

@@ -80,8 +80,6 @@ impl super::View for ContextMenus {
         ui.label("Right-click plot to edit it!");
         ui.horizontal(|ui| {
             self.example_plot(ui).context_menu(|ui| {
-                ui.set_min_width(220.0);
-
                 ui.menu_button("Plot", |ui| {
                     if ui.radio_value(&mut self.plot, Plot::Sin, "Sin").clicked()
                         || ui
@@ -188,6 +186,6 @@ impl ContextMenus {
                 ui.close_menu();
             }
         });
-        let _ = ui.button("Very long text for this item");
+        let _ = ui.button("Very long text for this item that should be wrapped");
     }
 }

crates/egui_demo_lib/src/demo/demo_app_windows.rs

@@ -334,7 +334,6 @@ fn file_menu_button(ui: &mut Ui) {
     }
 
     ui.menu_button("File", |ui| {
-        ui.set_min_width(220.0);
         ui.style_mut().wrap_mode = Some(egui::TextWrapMode::Extend);
 
         // On the web the browser controls the zoom

crates/emath/src/vec2.rs

@@ -131,6 +131,7 @@ impl Vec2 {
 
     pub const ZERO: Self = Self { x: 0.0, y: 0.0 };
     pub const INFINITY: Self = Self::splat(f32::INFINITY);
+    pub const NAN: Self = Self::splat(f32::NAN);
 
     #[inline(always)]
     pub const fn new(x: f32, y: f32) -> Self {

tests/test_size_pass/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "test_size_pass"
+version = "0.1.0"
+authors = ["Emil Ernerfeldt <emil.ernerfeldt@gmail.com>"]
+license = "MIT OR Apache-2.0"
+edition = "2021"
+rust-version = "1.76"
+publish = false
+
+[lints]
+workspace = true
+
+[features]
+wgpu = ["eframe/wgpu"]
+
+[dependencies]
+eframe = { workspace = true, features = [
+    "default",
+    "__screenshot", # __screenshot is so we can dump a screenshot using EFRAME_SCREENSHOT_TO
+] }
+env_logger = { version = "0.10", default-features = false, features = [
+    "auto-color",
+    "humantime",
+] }

tests/test_size_pass/src/main.rs

@@ -0,0 +1,25 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+#![allow(rustdoc::missing_crate_level_docs)] // it's a test
+
+use eframe::egui;
+
+fn main() -> eframe::Result<()> {
+    env_logger::init(); // Use `RUST_LOG=debug` to see logs.
+
+    let options = eframe::NativeOptions::default();
+    eframe::run_simple_native("My egui App", options, move |ctx, _frame| {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.label("The menu should be as wide as the widest button");
+            ui.menu_button("Click for menu", |ui| {
+                let _ = ui.button("Narrow").clicked();
+                let _ = ui.button("Very wide text").clicked();
+                let _ = ui.button("Narrow").clicked();
+            });
+
+            ui.label("Hover for tooltip").on_hover_ui(|ui| {
+                ui.label("A separator:");
+                ui.separator();
+            });
+        });
+    })
+}