[Security Solution][A11Y] add overflow scroll to timeline modal for small screens

x-pack/solutions/security/plugins/security_solution/public/timelines/components/timeline/index.tsx

@@ -41,6 +41,10 @@ const TimelineBody = styled.div`
   height: 100%;
   display: flex;
   flex-direction: column;
+  @media (max-width: 767px) {
+    height: fit-content;
+    min-height: 400px;
+  }
 `;
 
 export interface Props {

x-pack/solutions/security/plugins/security_solution/public/timelines/components/timeline/styles.tsx

@@ -20,6 +20,7 @@ export const TimelineContainer = styled.div.attrs(({ className = '' }) => ({
   display: flex;
   flex-direction: column;
   position: relative;
+  overflow: scroll;
 `;
 
 /**

x-pack/solutions/security/plugins/security_solution/public/timelines/components/timeline/tabs/shared/layout.tsx

@@ -20,6 +20,7 @@ export const StyledEuiFlyoutHeader = styled(EuiFlyoutHeader)`
   display: flex;
   flex-direction: column;
   padding: 0;
+  overflow: scroll;
 
   &.euiFlyoutHeader {
     ${({ theme }) => `padding: ${theme.eui.euiSizeS} 0 0 0;`}