--- a/devtools/client/themes/inspector.css
+++ b/devtools/client/themes/inspector.css
@@ -70,16 +70,24 @@
 }
 
 .event-header {
   display: flex;
   align-items: center;
   cursor: pointer;
 }
 
+.event-header:first-child {
+  border-width: 0;
+}
+
+.event-header:not(:first-child) {
+  border-width: 1px 0 0 0;
+}
+
 .event-tooltip-event-type,
 .event-tooltip-filename,
 .event-tooltip-attributes {
   -moz-margin-start: 0;
   flex-shrink: 0;
   cursor: pointer;
 }
 
@@ -103,18 +111,25 @@
 }
 
 .event-tooltip-debugger-icon:hover {
   opacity: 1;
 }
 
 .event-tooltip-content-box {
   display: none;
-  overflow: auto;
+  height: 100px;
+  overflow: hidden;
   -moz-margin-end: 0;
+  border: 1px solid var(--theme-splitter-color);
+  border-width: 1px 0 0 0;
+}
+
+.event-toolbox-content-box iframe {
+  height: 100%;
 }
 
 .event-tooltip-content-box[open] {
   display: block;
 }
 
 .event-tooltip-source-container {
   margin-top: 5px;