Submenu close delay not activated when mouse moves out of menu

[jasongrout]

Consider these two paths of mouse movement:

In the blue path, the submenu delays closing so you can get to the submenu item you are aiming for. However, if the mouse goes on the red path, the menu closes and a new top-level menu opens. This is frustrating if you are aiming for a submenu item.

Perhaps the delay at

lumino/packages/widgets/src/menu.ts

Lines 858 to 900 in e348a0d

	/**
	* Start the open timer, unless it is already pending.
	*/
	private _startOpenTimer(): void {
	if (this._openTimerID === 0) {
	this._openTimerID = window.setTimeout(() => {
	this._openTimerID = 0;
	this._openChildMenu();
	}, Private.TIMER_DELAY);
	}
	}
	/**
	* Start the close timer, unless it is already pending.
	*/
	private _startCloseTimer(): void {
	if (this._closeTimerID === 0) {
	this._closeTimerID = window.setTimeout(() => {
	this._closeTimerID = 0;
	this._closeChildMenu();
	}, Private.TIMER_DELAY);
	}
	}
	/**
	* Cancel the open timer, if the timer is pending.
	*/
	private _cancelOpenTimer(): void {
	if (this._openTimerID !== 0) {
	clearTimeout(this._openTimerID);
	this._openTimerID = 0;
	}
	}
	/**
	* Cancel the close timer, if the timer is pending.
	*/
	private _cancelCloseTimer(): void {
	if (this._closeTimerID !== 0) {
	clearTimeout(this._closeTimerID);
	this._closeTimerID = 0;
	}
	}

should work if the mouse pointed strays outside of the menu block as well?

See jupyterlab/jupyterlab#9201 for more context.