Put a scrollbar on the collections sidebar

A tree longer than the panel looked, to a mouse, like the whole tree.
Same shape as the develop column: the Flickable fills a Rectangle that
takes its stretch, and a ScrollBar is drawn over its right edge. It is
drawn only when the tree overflows.

docs/gestures.md

@@ -407,7 +407,7 @@ Double-click is what a file manager and a Lightroom panel use for the same thing
 
 Under the trash because the trash is where the spare copies go, and only while the catalog holds any: a row that is always there and usually empty is noise.
 
-<sub>`ui/dr-ui/ui/collections.slint:870`</sub>
+<sub>`ui/dr-ui/ui/collections.slint:884`</sub>
 
 ## Duplicate originals
 

ui/dr-ui/ui/collections.slint

@@ -34,7 +34,7 @@
 import { Theme } from "theme.slint";
 import { Duplicates } from "duplicates.slint";
 import { Keys } from "keys.slint";
-import { Button, Icon } from "widgets.slint";
+import { Button, Icon, ScrollBar } from "widgets.slint";
 
 // One row of the collection tree.
 export struct CollectionRow {
@@ -673,8 +673,11 @@ export component CollectionsPanel inherits Rectangle {
         }
 
         // --- the tree -----------------------------------------------------
-        Flickable {
+        Rectangle {
                 vertical-stretch: 1;
+            tree-flick := Flickable {
+                width: parent.width;
+                height: parent.height;
                 viewport-height: root.rows.length * (Theme.row-height + 2px);
 
                 // Wrapped rather than made draggable from the inside: the grid's
@@ -746,6 +749,17 @@ export component CollectionsPanel inherits Rectangle {
                 }
             }
 
+            // A bar for a pointer: a tree of collections longer than the panel
+            // otherwise looks, to a mouse, like the whole tree.
+            ScrollBar {
+                x: parent.width - self.width;
+                y: 0px;
+                viewport-y <=> tree-flick.viewport-y;
+                viewport-height: tree-flick.viewport-height;
+                visible-height: tree-flick.height;
+            }
+        }
+
         // TRACES: FR-NC-6a
         // Keep the scoped collection on this device.
         //