Fix sticky header in diff view (#23554) (#23568)

Backport #23554 by @silverwind

Ressurection of #23549.

Fix regression
#23513 (comment)
from #23271.
The previous sticky CSS did assume the content is always 2 rows, but
since that PR, it's single-row above 993px width.
Adjust the sticky offset to match and add a small tweak that hides
content behind the `border-radius`.

Single row:
<img width="1264" alt="Screenshot 2023-03-17 at 21 33 05"
src="https://user-images.githubusercontent.com/115237/226034050-a04b131d-fd3f-45c0-bc72-413738a59825.png">

Double row:
<img width="1243" alt="Screenshot 2023-03-17 at 21 32 53"
src="https://user-images.githubusercontent.com/115237/226034163-2f1c6aa9-fc72-432f-bc46-9a7119da8677.png">

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/repository.css

@@ -1611,12 +1611,14 @@
  padding: 7px 0;
  background: var(--color-body);
  line-height: 30px;
+ height: 47px; /* match .ui.attached.header.diff-file-header.sticky-2nd-row */
 }
 
 @media (max-width: 991px) {
  .repository .diff-detail-box {
  flex-direction: column;
  align-items: flex-start;
+ height: 77px; /* match .ui.attached.header.diff-file-header.sticky-2nd-row */
  }
 }
 
@@ -1630,7 +1632,7 @@
  position: sticky;
  top: 0;
  z-index: 8;
- border-bottom: 1px solid var(--color-secondary);
+ border-bottom: none;
  padding-left: 2px;
  padding-right: 2px;
  margin-left: -1px;
@@ -3318,10 +3320,16 @@ td.blob-excerpt {
 
 .ui.attached.header.diff-file-header.sticky-2nd-row {
  position: sticky;
- top: 77px;
+ top: 47px; /* match .repository .diff-detail-box */
  z-index: 7;
 }
 
+@media (max-width: 991px) {
+ .ui.attached.header.diff-file-header.sticky-2nd-row {
+ top: 77px; /* match .repository .diff-detail-box */
+ }
+}
+
 @media (max-width: 480px) {
  .ui.attached.header.diff-file-header.sticky-2nd-row {
  position: static;