fixes #93965

src/vs/base/browser/ui/splitview/paneview.ts

@@ -31,6 +31,7 @@ export interface IPaneStyles {
 	headerForeground?: Color;
 	headerBackground?: Color;
 	headerBorder?: Color;
+	leftBorder?: Color;
 }
 
 /**
@@ -243,6 +244,8 @@ export abstract class Pane extends Disposable implements IView {
 	style(styles: IPaneStyles): void {
 		this.styles = styles;
 
+		this.element.style.borderLeft = this.styles.leftBorder && this.orientation === Orientation.HORIZONTAL ? `1px solid ${this.styles.leftBorder}` : '';
+
 		if (!this.header) {
 			return;
 		}
@@ -261,7 +264,7 @@ export abstract class Pane extends Disposable implements IView {
 
 		this.header.style.color = this.styles.headerForeground ? this.styles.headerForeground.toString() : '';
 		this.header.style.backgroundColor = this.styles.headerBackground ? this.styles.headerBackground.toString() : '';
-		this.header.style.borderTop = this.styles.headerBorder ? `1px solid ${this.styles.headerBorder}` : '';
+		this.header.style.borderTop = this.styles.headerBorder && this.orientation === Orientation.VERTICAL ? `1px solid ${this.styles.headerBorder}` : '';
 		this._dropBackground = this.styles.dropBackground;
 	}
 

src/vs/workbench/browser/parts/views/media/paneviewlet.css

@@ -6,6 +6,9 @@
 .monaco-pane-view .split-view-view:first-of-type > .pane > .pane-header {
 	border-top: none !important; /* less clutter: do not show any border for first views in a pane */
 }
+.monaco-pane-view .split-view-view:first-of-type > .pane {
+	border-left: none !important; /* less clutter: do not show any border for first views in a pane */
+}
 
 .monaco-pane-view .pane > .pane-header {
 	position: relative;

src/vs/workbench/browser/parts/views/viewPaneContainer.ts

@@ -8,7 +8,7 @@ import * as nls from 'vs/nls';
 import { Event, Emitter } from 'vs/base/common/event';
 import { ColorIdentifier, activeContrastBorder } from 'vs/platform/theme/common/colorRegistry';
 import { attachStyler, IColorMapping, attachButtonStyler, attachLinkStyler, attachProgressBarStyler } from 'vs/platform/theme/common/styler';
-import { SIDE_BAR_DRAG_AND_DROP_BACKGROUND, SIDE_BAR_SECTION_HEADER_FOREGROUND, SIDE_BAR_SECTION_HEADER_BACKGROUND, SIDE_BAR_SECTION_HEADER_BORDER, PANEL_BACKGROUND, SIDE_BAR_BACKGROUND, EDITOR_DRAG_AND_DROP_BACKGROUND } from 'vs/workbench/common/theme';
+import { SIDE_BAR_DRAG_AND_DROP_BACKGROUND, SIDE_BAR_SECTION_HEADER_FOREGROUND, SIDE_BAR_SECTION_HEADER_BACKGROUND, SIDE_BAR_SECTION_HEADER_BORDER, PANEL_BACKGROUND, SIDE_BAR_BACKGROUND, EDITOR_DRAG_AND_DROP_BACKGROUND, PANEL_BORDER } from 'vs/workbench/common/theme';
 import { append, $, trackFocus, toggleClass, EventType, isAncestor, Dimension, addDisposableListener, removeClass, addClass } from 'vs/base/browser/dom';
 import { IDisposable, combinedDisposable, dispose, toDisposable, Disposable, DisposableStore } from 'vs/base/common/lifecycle';
 import { firstIndex } from 'vs/base/common/arrays';
@@ -55,6 +55,7 @@ export interface IPaneColors extends IColorMapping {
 	headerForeground?: ColorIdentifier;
 	headerBackground?: ColorIdentifier;
 	headerBorder?: ColorIdentifier;
+	leftBorder?: ColorIdentifier;
 }
 
 export interface IViewPaneOptions extends IPaneOptions {
@@ -1171,6 +1172,7 @@ export class ViewPaneContainer extends Component implements IViewPaneContainer {
 			headerForeground: SIDE_BAR_SECTION_HEADER_FOREGROUND,
 			headerBackground: SIDE_BAR_SECTION_HEADER_BACKGROUND,
 			headerBorder: SIDE_BAR_SECTION_HEADER_BORDER,
+			leftBorder: PANEL_BORDER,
 			dropBackground: SIDE_BAR_DRAG_AND_DROP_BACKGROUND
 		}, pane);
 		const disposable = combinedDisposable(onDidFocus, onDidChangeTitleArea, paneStyler, onDidChange, onDidChangeVisibility);