Prevent ScrollArea:s from becoming tiny (#1255)

Closes https://github.com/emilk/egui/issues/1097

CHANGELOG.md

@@ -55,11 +55,11 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 * Renamed `Plot::custom_label_func` to `Plot::label_formatter` ([#1235](https://github.com/emilk/egui/pull/1235)).
 * Tooltips that don't fit the window don't flicker anymore ([#1240](https://github.com/emilk/egui/pull/1240)).
 * `Areas::layer_id_at` ignores non interatable layers (i.e. Tooltips) ([#1240](https://github.com/emilk/egui/pull/1240)).
+* `ScrollArea`:s will not shrink below a certain minimum size, set by `min_scrolled_width/min_scrolled_height` ([1255](https://github.com/emilk/egui/pull/1255)).
 
 ### Fixed 🐛
 * Context menus now respects the theme ([#1043](https://github.com/emilk/egui/pull/1043)).
 * Plot `Orientation` was not public, although fields using this type were ([#1130](https://github.com/emilk/egui/pull/1130)).
-* Fixed `enable_drag` for Windows ([#1108](https://github.com/emilk/egui/pull/1108)).
 * Calling `Context::set_pixels_per_point` before the first frame will now work.
 * Tooltips that don't fit the window don't flicker anymore ([#1240](https://github.com/emilk/egui/pull/1240)).
 * Scroll areas now follow text cursor ([1252](https://github.com/emilk/egui/pull/1252)).

eframe/CHANGELOG.md

@@ -12,6 +12,7 @@ NOTE: [`egui_web`](../egui_web/CHANGELOG.md), [`egui-winit`](../egui-winit/CHANG
 * Fix horizontal scrolling direction on Linux.
 * Added `App::on_exit_event` ([#1038](https://github.com/emilk/egui/pull/1038))
 * Added `NativeOptions::initial_window_pos`.
+* Fixed `enable_drag` for Windows ([#1108](https://github.com/emilk/egui/pull/1108)).
 * Shift-scroll will now result in horizontal scrolling on all platforms ([#1136](https://github.com/emilk/egui/pull/1136)).
 * Log using the `tracing` crate. Log to stdout by adding `tracing_subscriber::fmt::init();` to your `main` ([#1192](https://github.com/emilk/egui/pull/1192)).
 * Expose all parts of the location/url in `frame.info().web_info` ([#1258](https://github.com/emilk/egui/pull/1258)).

egui/src/containers/scroll_area.rs

@@ -81,6 +81,7 @@ pub struct ScrollArea {
     has_bar: [bool; 2],
     auto_shrink: [bool; 2],
     max_size: Vec2,
+    min_scrolled_size: Vec2,
     always_show_scroll: bool,
     id_source: Option<Id>,
     offset_x: Option<f32>,
@@ -123,6 +124,7 @@ impl ScrollArea {
             has_bar,
             auto_shrink: [true; 2],
             max_size: Vec2::INFINITY,
+            min_scrolled_size: Vec2::splat(64.0),
             always_show_scroll: false,
             id_source: None,
             offset_x: None,
@@ -152,6 +154,28 @@ impl ScrollArea {
         self
     }
 
+    /// The minimum width of a horizontal scroll area which requires scroll bars.
+    ///
+    /// The `ScrollArea` will only become smaller than this if the content is smaller than this
+    /// (and so we don't require scroll bars).
+    ///
+    /// Default: `64.0`.
+    pub fn min_scrolled_width(mut self, min_scrolled_width: f32) -> Self {
+        self.min_scrolled_size.x = min_scrolled_width;
+        self
+    }
+
+    /// The minimum height of a vertical scroll area which requires scroll bars.
+    ///
+    /// The `ScrollArea` will only become smaller than this if the content is smaller than this
+    /// (and so we don't require scroll bars).
+    ///
+    /// Default: `64.0`.
+    pub fn min_scrolled_height(mut self, min_scrolled_height: f32) -> Self {
+        self.min_scrolled_size.y = min_scrolled_height;
+        self
+    }
+
     /// If `false` (default), the scroll bar will be hidden when not needed/
     /// If `true`, the scroll bar will always be displayed even if not needed.
     pub fn always_show_scroll(mut self, always_show_scroll: bool) -> Self {
@@ -288,6 +312,7 @@ impl ScrollArea {
             has_bar,
             auto_shrink,
             max_size,
+            min_scrolled_size,
             always_show_scroll,
             id_source,
             offset_x,
@@ -329,27 +354,39 @@ impl ScrollArea {
 
         let outer_size = available_outer.size().at_most(max_size);
 
-        let inner_size = outer_size - current_bar_use;
+        let inner_size = {
+            let mut inner_size = outer_size - current_bar_use;
+
+            // Don't go so far that we shrink to zero.
+            // In particular, if we put a `ScrollArea` inside of a `ScrollArea`, the inner
+            // one shouldn't collapse into nothingness.
+            // See https://github.com/emilk/egui/issues/1097
+            for d in 0..2 {
+                if has_bar[d] {
+                    inner_size[d] = inner_size[d].max(min_scrolled_size[d]);
+                }
+            }
+            inner_size
+        };
+
         let inner_rect = Rect::from_min_size(available_outer.min, inner_size);
 
-        let mut inner_child_max_size = inner_size;
+        let mut content_max_size = inner_size;
 
         if true {
             // Tell the inner Ui to *try* to fit the content without needing to scroll,
-            // i.e. better to wrap text than showing a horizontal scrollbar!
+            // i.e. better to wrap text and shrink images than showing a horizontal scrollbar!
         } else {
             // Tell the inner Ui to use as much space as possible, we can scroll to see it!
             for d in 0..2 {
                 if has_bar[d] {
-                    inner_child_max_size[d] = f32::INFINITY;
+                    content_max_size[d] = f32::INFINITY;
                 }
             }
         }
 
-        let mut content_ui = ui.child_ui(
+        let content_max_rect = Rect::from_min_size(inner_rect.min - state.offset, content_max_size);
-            Rect::from_min_size(inner_rect.min - state.offset, inner_child_max_size),
+        let mut content_ui = ui.child_ui(content_max_rect, *ui.layout());
-            *ui.layout(),
-        );
         let mut content_clip_rect = inner_rect.expand(ui.visuals().clip_rect_margin);
         content_clip_rect = content_clip_rect.intersect(ui.clip_rect());
         // Nice handling of forced resizing beyond the possible: