auto-shrink ScrollArea to content size

Unless the user disables it with `ScollArea::auto_shrink([false; 2])`.

CHANGELOG.md

@@ -25,6 +25,7 @@ NOTE: [`eframe`](eframe/CHANGELOG.md), [`egui_web`](egui_web/CHANGELOG.md), [`eg
 * MSRV (Minimum Supported Rust Version) is now `1.54.0`.
 * By default, `DragValue`:s no longer show a tooltip when hovered. Change with `Style::explanation_tooltips`.
 * Smaller and nicer color picker.
+* `ScrollArea` will auto-shrink to content size unless told otherwise using `ScollArea::auto_shrink`.
 
 ### Fixed 🐛
 * Fix wrongly sized multiline `TextEdit` in justified layouts.

egui/src/containers/scroll_area.rs

@@ -341,14 +341,14 @@ impl ScrollArea {
 
             let y_min = ui.max_rect().top() + min_row as f32 * row_height_with_spacing;
             let y_max = ui.max_rect().top() + max_row as f32 * row_height_with_spacing;
-            let mut viewport_ui = ui.child_ui(
-                Rect::from_x_y_ranges(ui.max_rect().x_range(), y_min..=y_max),
-                *ui.layout(),
-            );
 
+            let rect = Rect::from_x_y_ranges(ui.max_rect().x_range(), y_min..=y_max);
+
+            ui.allocate_ui_at_rect(rect, |viewport_ui| {
                 viewport_ui.skip_ahead_auto_ids(min_row); // Make sure we get consistent IDs.
-
+                add_contents(viewport_ui, min_row..max_row)
-            add_contents(&mut viewport_ui, min_row..max_row)
+            })
+            .inner
         })
     }
 
@@ -389,10 +389,9 @@ impl Prepared {
 
         let content_size = content_ui.min_size();
 
-        // We take the scroll target so only this ScrollArea will use it.
-
         for d in 0..2 {
             if has_bar[d] {
+                // We take the scroll target so only this ScrollArea will use it:
                 let scroll_target = content_ui.ctx().frame_state().scroll_target[d].take();
                 if let Some((scroll, align)) = scroll_target {
                     let center_factor = align.to_factor();
@@ -412,19 +411,15 @@ impl Prepared {
         }
 
         let inner_rect = {
+            // At this point this is the available size for the inner rect.
             let mut inner_size = inner_rect.size();
 
             for d in 0..2 {
-                inner_size[d] = if has_bar[d] {
+                inner_size[d] = match (has_bar[d], auto_shrink[d]) {
-                    if auto_shrink[d] {
+                    (true, true) => inner_size[d].min(content_size[d]), // shrink scroll area if content is small
-                        inner_size[d].min(content_size[d]) // shrink scroll area if content is small
+                    (true, false) => inner_size[d], // let scroll area be larger than content; fill with blank space
-                    } else {
+                    (false, true) => content_size[d], // Follow the content (expand/contract to fit it).
-                        inner_size[d] // let scroll area be larger than content; fill with blank space
+                    (false, false) => inner_size[d].max(content_size[d]), // Expand to fit content
-                    }
-                } else if inner_size[d].is_finite() {
-                    inner_size[d].max(content_size[d]) // Expand to fit content
-                } else {
-                    content_size[d] // ScrollArea is in an infinitely sized parent; take size of parent
                 };
             }
 

egui_demo_lib/src/apps/demo/scrolling.rs

@@ -32,7 +32,7 @@ impl super::Demo for Scrolling {
             .open(open)
             .resizable(false)
             .show(ctx, |ui| {
-                use super::View;
+                use super::View as _;
                 self.ui(ui);
             });
     }
@@ -77,12 +77,17 @@ fn huge_content_lines(ui: &mut egui::Ui) {
     let text_style = TextStyle::Body;
     let row_height = ui.fonts()[text_style].row_height();
     let num_rows = 10_000;
-    ScrollArea::vertical().show_rows(ui, row_height, num_rows, |ui, row_range| {
+    ScrollArea::vertical().auto_shrink([false; 2]).show_rows(
+        ui,
+        row_height,
+        num_rows,
+        |ui, row_range| {
             for row in row_range {
                 let text = format!("This is row {}/{}", row + 1, num_rows);
                 ui.label(text);
             }
-    });
+        },
+    );
 }
 
 fn huge_content_painter(ui: &mut egui::Ui) {
@@ -94,13 +99,17 @@ fn huge_content_painter(ui: &mut egui::Ui) {
     let row_height = ui.fonts()[text_style].row_height() + ui.spacing().item_spacing.y;
     let num_rows = 10_000;
 
-    ScrollArea::vertical().show_viewport(ui, |ui, viewport| {
+    ScrollArea::vertical()
+        .auto_shrink([false; 2])
+        .show_viewport(ui, |ui, viewport| {
             ui.set_height(row_height * num_rows as f32);
 
             let first_item = (viewport.min.y / row_height).floor().at_least(0.0) as usize;
             let last_item = (viewport.max.y / row_height).ceil() as usize + 1;
             let last_item = last_item.at_most(num_rows);
 
+            let mut used_rect = Rect::NOTHING;
+
             for i in first_item..last_item {
                 let indentation = (i % 100) as f32;
                 let x = ui.min_rect().left() + indentation;
@@ -111,14 +120,17 @@ fn huge_content_painter(ui: &mut egui::Ui) {
                     num_rows,
                     indentation
                 );
-            ui.painter().text(
+                let text_rect = ui.painter().text(
                     pos2(x, y),
                     Align2::LEFT_TOP,
                     text,
                     text_style,
                     ui.visuals().text_color(),
                 );
+                used_rect = used_rect.union(text_rect);
             }
+
+            ui.allocate_rect(used_rect, Sense::hover()); // make sure it is visible!
         });
 }
 
@@ -184,7 +196,9 @@ impl super::View for ScrollTo {
             scroll_bottom |= ui.button("Scroll to bottom").clicked();
         });
 
-        let mut scroll_area = ScrollArea::vertical().max_height(200.0);
+        let mut scroll_area = ScrollArea::vertical()
+            .max_height(200.0)
+            .auto_shrink([false; 2]);
         if go_to_scroll_offset {
             scroll_area = scroll_area.scroll_offset(self.offset);
         }