Fix flickering when clearing edits session (#239171)

This layout code needs to be cleaned up. Basically the view renders without the working set for an instant before the new model is initialized, then the working set comes back. The welcome view tries to center itself vertically, that's where the flicker comes from. I already fixed the same issue for followups in the chat view, we reserve some space for them. Applying the same to the working set's height.

src/vs/workbench/contrib/chat/browser/chatInputPart.ts

@@ -256,6 +256,11 @@ export class ChatInputPart extends Disposable implements IHistoryNavigationWidge
 		return this._followupsHeight;
 	}
 
+	private _editSessionWidgetHeight: number = 0;
+	get editSessionWidgetHeight() {
+		return this._editSessionWidgetHeight;
+	}
+
 	private _inputEditor!: CodeEditorWidget;
 	private _inputEditorElement!: HTMLElement;
 
@@ -1514,6 +1519,7 @@ export class ChatInputPart extends Disposable implements IHistoryNavigationWidge
 
 		this._inputPartHeight = data.inputPartVerticalPadding + data.followupsHeight + inputEditorHeight + data.inputEditorBorder + data.attachmentsHeight + data.toolbarsHeight + data.chatEditingStateHeight;
 		this._followupsHeight = data.followupsHeight;
+		this._editSessionWidgetHeight = data.chatEditingStateHeight;
 
 		const initialEditorScrollWidth = this._inputEditor.getScrollWidth();
 		const newEditorWidth = width - data.inputPartHorizontalPadding - data.editorBorder - data.inputPartHorizontalPaddingInside - data.toolbarsWidth - data.sideToolbarWidth;

src/vs/workbench/contrib/chat/browser/chatWidget.ts

@@ -1208,10 +1208,15 @@ export class ChatWidget extends Disposable implements IChatWidget {
 		this.tree.layout(listHeight, width);
 		this.tree.getHTMLElement().style.height = `${listHeight}px`;
 
-		// Push the welcome message down so it doesn't change position when followups appear
-		const followupsOffset = this.viewOptions.renderFollowups ? Math.max(100 - this.inputPart.followupsHeight, 0) : 0;
-		this.welcomeMessageContainer.style.height = `${listHeight - followupsOffset}px`;
-		this.welcomeMessageContainer.style.paddingBottom = `${followupsOffset}px`;
+		// Push the welcome message down so it doesn't change position when followups or working set appear
+		let extraOffset: number = 0;
+		if (this.viewOptions.renderFollowups) {
+			extraOffset = Math.max(100 - this.inputPart.followupsHeight, 0);
+		} else if (this.viewOptions.enableWorkingSet) {
+			extraOffset = Math.max(100 - this.inputPart.editSessionWidgetHeight, 0);
+		}
+		this.welcomeMessageContainer.style.height = `${listHeight - extraOffset}px`;
+		this.welcomeMessageContainer.style.paddingBottom = `${extraOffset}px`;
 		this.renderer.layout(width);
 
 		const lastItem = this.viewModel?.getItems().at(-1);