Make sure the scroll bar is never outside the screen rectangle (#475)

* Make sure the scroll bar is never outside the screen rectangle

This is an alternative attempt to fix the bug mentioned in
https://github.com/emilk/egui/pull/392

egui expects that the container can always be made wider,
which is true for all egui Ui:s, but not true for the outer
frame/chrome that egui ultimately needs to sit within.

* Clamp scroll to screen rect rather than available rect

* Fix scrollbar spacing when shrinking too small

* Update changelog

CHANGELOG.md

@@ -28,6 +28,9 @@ NOTE: [`eframe`](eframe/CHANGELOG.md), [`egui_web`](egui_web/CHANGELOG.md) and [
 * `TopPanel::top` is now `TopBottomPanel::top`.
 * `SidePanel::left` no longet takes the default width by argument, but by a builder call.
 
+### Fixed 🐛
+* Fix invisible scroll bar when native window is too narrow for egui.
+
 
 ## 0.12.0 - 2021-05-10 - Multitouch, user memory, window pivots, and improved plots
 

egui/src/containers/scroll_area.rs

@@ -267,6 +267,7 @@ impl Prepared {
             state.offset.y = offset_y + spacing;
         }
 
+        let inner_rect = {
             let width = if inner_rect.width().is_finite() {
                 inner_rect.width().max(content_size.x) // Expand width to fit content
             } else {
@@ -274,7 +275,19 @@ impl Prepared {
                 content_size.x
             };
 
-        let inner_rect = Rect::from_min_size(inner_rect.min, vec2(width, inner_rect.height()));
+            let mut inner_rect =
+                Rect::from_min_size(inner_rect.min, vec2(width, inner_rect.height()));
+
+            // The window that egui sits in can't be expanded by egui, so we need to respect it:
+            let max_x = ui.input().screen_rect().right()
+                - current_scroll_bar_width
+                - ui.spacing().item_spacing.x;
+            inner_rect.max.x = inner_rect.max.x.at_most(max_x);
+            // TODO: when we support it, we should maybe auto-enable
+            // horizontal scrolling if this limit is reached
+
+            inner_rect
+        };
 
         let outer_rect = Rect::from_min_size(
             inner_rect.min,