src/vs/workbench/browser/parts/editor/editorControl.ts/EditorControl/doSetActiveEditorPane
class EditorControl:	private doSetActiveEditorPane(editorPane: EditorPane | null) {
		this._activeEditorPane = editorPane;

		// Clear out previous active editor pane listeners
		this.activeEditorPaneDisposables.clear();

		// Listen to editor pane changes
		if (editorPane) {
			this.activeEditorPaneDisposables.add(editorPane.onDidChangeSizeConstraints(e => this._onDidChangeSizeConstraints.fire(e)));
			this.activeEditorPaneDisposables.add(editorPane.onDidFocus(() => this._onDidFocus.fire()));
		}

		// Indicate that size constraints could have changed due to new editor
		this._onDidChangeSizeConstraints.fire(undefined);
	}