right-clicking file node in the explorer does not populate the right context menu

[elaihau]

Description

Right clicking the Explorer populates two types of context menus:

1. context menu when right clicking a file node
   

2. context menu when right clicking the empty area
   

However, Theia does not always behave this way. Sometimes right clicking a file node can populate a "type-2" (see screenshot above) context menu.

Reproduction Steps

As the GIF shows, to reproduce perform the following operations

• left click a file node to open the file in editor
• close the editor opened in step 1
• left click the empty area in the main panel, make sure the file node highlighted in Step 1 and 2 turns gray
• refresh the browser
• right click the file node high lighted in gray color

expected: the context menu for file nodes should be populated
what happened: the context menu for the empty area was populated

This also happened to files newly created in the tree. I just couldn't find a reliable way to reproduce it atm.

OS and Theia version:

Ubuntu 18.04, example browser, Chrome

master branch f255f5a