Make the scrollbar always visible in lookback dropdown (#3048)

## Which problem is this PR solving?
- Fixes #469 
<img width="442" height="351" alt="Screenshot 2025-08-29 211551"
src="https://github.com/user-attachments/assets/18fc86b8-ecec-4c8c-adb8-6a1a359594ae"
/>


## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits
- [ ] I have added unit tests for the new functionality
- [x] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `npm run lint` and `npm run test`

Signed-off-by: Parship Chowdhury <i.am.parship@gmail.com>

Author: Parship Chowdhury

packages/jaeger-ui/src/components/SearchTracePage/SearchForm.css

@@ -58,3 +58,22 @@ limitations under the License.
 .SearchForm--tagsHintEg {
   color: teal;
 }
+
+.ant-select-dropdown .rc-virtual-list-holder {
+  overflow-y: auto !important;
+}
+
+/* ensure scrollbar is always visible, not just on hover */
+.ant-select-dropdown::-webkit-scrollbar,
+.ant-select-dropdown .rc-virtual-list::-webkit-scrollbar,
+.ant-select-dropdown .rc-virtual-list-holder::-webkit-scrollbar {
+  width: 8px;
+  background-color: #f1f1f1;
+}
+
+.ant-select-dropdown::-webkit-scrollbar-thumb,
+.ant-select-dropdown .rc-virtual-list::-webkit-scrollbar-thumb,
+.ant-select-dropdown .rc-virtual-list-holder::-webkit-scrollbar-thumb {
+  background-color: #292929;
+  border-radius: 4px;
+}