Fix cell editor highlight inset

src/vs/workbench/contrib/notebook/browser/media/notebook.css

@@ -308,6 +308,19 @@
 	visibility: visible;
 }
 
+.monaco-workbench .part.editor > .content .notebook-editor .monaco-list-row.cell-editor-focus .cell-editor-part:before {
+	z-index: 5;
+	content: "";
+	right: 0px;
+	left: 0px;
+	top: 0px;
+	bottom: 0px;
+	outline-offset: -1px;
+	display: block;
+	position: absolute;
+	pointer-events: none;
+}
+
 .monaco-workbench .part.editor > .content .notebook-editor .monaco-list .monaco-list-row.cell-dragover .notebook-cell-insertion-indicator-top {
 	opacity: 1;
 }

src/vs/workbench/contrib/notebook/browser/notebookEditor.ts

@@ -1028,7 +1028,7 @@ registerThemingParticipant((theme, collector) => {
 		collector.addRule(`.monaco-workbench .part.editor > .content .notebook-editor .monaco-list-row.focused .notebook-cell-focus-indicator { border-color: ${focusedCellIndicatorColor}; }`);
 		collector.addRule(`.monaco-workbench .part.editor > .content .notebook-editor .monaco-list-row .notebook-cell-focus-indicator { border-color: ${focusedCellIndicatorColor}; }`);
 		collector.addRule(`.monaco-workbench .part.editor > .content .notebook-editor .monaco-list-row .notebook-cell-insertion-indicator-top { background-color: ${focusedCellIndicatorColor}; }`);
-		collector.addRule(`.monaco-workbench .part.editor > .content .notebook-editor .monaco-list-row.cell-editor-focus .cell-editor-part { outline: solid 1px ${focusedCellIndicatorColor}; }`);
+		collector.addRule(`.monaco-workbench .part.editor > .content .notebook-editor .monaco-list-row.cell-editor-focus .cell-editor-part:before { outline: solid 1px ${focusedCellIndicatorColor}; }`);
 	}
 
 	// Cell Margin