fix(ui): prevent selection checkboxes from creating phantom scroll height

Add `position: relative` to `.bui-Table` so it becomes the containing
block for react-aria's absolutely positioned hidden checkbox inputs.

Without this, the inputs position themselves relative to a distant
ancestor, extending its scrollable area and creating phantom scroll
height.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

.changeset/hot-socks-cross.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed Table with row selection creating phantom scroll height on ancestor elements by establishing a containing block for visually-hidden checkbox inputs.
+
+**Affected components:** Table, TableRoot

packages/ui/src/components/Table/Table.module.css

@@ -31,6 +31,8 @@
   }
 
   .bui-Table {
+    /* Establishes containing block for react-aria's absolutely positioned hidden checkbox inputs, preventing them from escaping to a distant ancestor and creating phantom scroll height. */
+    position: relative;
     width: 100%;
     caption-side: bottom;
     border-collapse: collapse;