get toolbar width only once

fixes #39185

src/vs/workbench/browser/parts/panel/panelPart.ts

@@ -31,6 +31,7 @@ import { ToggleCompositePinnedAction } from 'vs/workbench/browser/parts/composit
 import { StandardMouseEvent } from 'vs/base/browser/mouseEvent';
 import { dispose, IDisposable } from 'vs/base/common/lifecycle';
 import { IBadge } from 'vs/workbench/services/activity/common/activity';
+import { memoize } from 'vs/base/common/decorators';
 
 export class PanelPart extends CompositePart<Panel> implements IPanelService {
 
@@ -234,12 +235,17 @@ export class PanelPart extends CompositePart<Panel> implements IPanelService {
 			let availableWidth = this.dimension.width - 8; // take padding into account
 			if (this.toolBar) {
 				// adjust height for global actions showing
-				availableWidth = Math.max(PanelPart.MIN_COMPOSITE_BAR_WIDTH, availableWidth - this.toolBar.getContainer().getHTMLElement().offsetWidth);
+				availableWidth = Math.max(PanelPart.MIN_COMPOSITE_BAR_WIDTH, availableWidth - this.toolbarWidth);
 			}
 			this.compositeBar.layout(new Dimension(availableWidth, this.dimension.height));
 		}
 	}
 
+	@memoize
+	private get toolbarWidth(): number {
+		return this.toolBar.getContainer().getHTMLElement().offsetWidth;
+	}
+
 	public shutdown(): void {
 		// Persist Hidden State
 		this.compositeBar.store();