Keep tool call headers pinned while content scrolls

src/styles/components.css

@@ -768,6 +768,7 @@ button.button-primary {
   scrollbar-width: thin;
   scrollbar-color: var(--border-base) transparent;
   scrollbar-gutter: stable both-edges;
+  position: relative;
 }
 
 .tool-call-markdown-large {
@@ -783,7 +784,8 @@ button.button-primary {
 .tool-call-markdown .code-block-header {
   position: sticky;
   top: 0;
-  z-index: 1;
+  z-index: 2;
+  box-shadow: 0 1px 0 var(--border-base);
 }
 
 .tool-call-markdown .markdown-code-block pre {

src/styles/markdown.css

@@ -126,7 +126,6 @@
   position: relative;
   margin: 12px 0;
   border-radius: 6px;
-  overflow: hidden;
   background-color: var(--surface-muted);
   border: 1px solid var(--border-base);
 }
@@ -138,6 +137,8 @@
   padding: 8px 12px;
   background-color: var(--surface-secondary);
   border-bottom: 1px solid var(--border-base);
+  border-top-left-radius: 6px;
+  border-top-right-radius: 6px;
 }
 
 .code-block-language {
@@ -182,6 +183,8 @@
   padding: 12px !important;
   overflow-x: auto;
   background-color: transparent !important;
+  border-bottom-left-radius: 6px;
+  border-bottom-right-radius: 6px;
 }
 
 .markdown-code-block code {