onAfterAttach being run when widget not attached

[colin-grant-work]

Bug Description:

Recently I have been seeing a number of messages like this in my console:

Many have to do with code introduced in #9935

theia/packages/core/src/browser/view-container.ts

Line 931 in 16f3309

Widget.attach(this.toolbar, this.header);

But at least one (the one in the screenshot above) points here:

theia/packages/core/src/browser/tree/tree-widget.tsx

Line 1054 in 16f3309

Widget.attach(this.searchBox, this.node.parentElement!);

Which predates #9935 and probably wasn't affected by it (🤞).

What's happening in that somehow, when the onAfterAttach handlers are run, the widget's node is not contained in document.body

And that triggers this error in Phosphor:

https://github.com/phosphorjs/phosphor/blob/9f5e11025b62d2c4a6fb59e2681ae1ed323dcde4/packages/widgets/src/widget.ts#L1003-L1005

Adding a simple check like if (this.isAttached) doesn't seem to fix the problem, and it's very strange that an onAfterAttach handler is running but the widget claims that it is not attached.

Steps to Reproduce:

1. Start the application (browser mode)
2. Check the console
3. Observe a number of Host is not attached errors logged.

Additional Information

• Operating System: RHEL7
• Theia Version: 16f3309

[colin-grant-work]

There are a number of places where we run code like this:

theia/packages/core/src/browser/view-container.ts

Lines 1044 to 1050 in 16f3309

	MessageLoop.sendMessage(widget, Widget.Msg.BeforeDetach);
	this.parent!.node.removeChild(widget.node);
	MessageLoop.sendMessage(widget, Widget.Msg.AfterDetach);
	MessageLoop.sendMessage(widget, Widget.Msg.BeforeAttach);
	this.parent!.node.insertBefore(widget.node, this.handles[toIndex]);
	MessageLoop.sendMessage(widget, Widget.Msg.AfterAttach);

Which is basically the same as Widget.detach and Widget.attach, except it doesn't check for whether the widget can be attached / detached as far as Phosphor is concerned. It seems like that probably points to areas where we're doing things that our widgets don't expect to be done.