Tip not showing in read-only

[freakdran]

monaco-editor version: 0.15.6
Browser: Chrome
OS: OSX

Steps or JS usage snippet reproducing the issue:

1. Create a readonly editor with some preset code in it
2. Press "Delete Left/Right" (or "Delete/Backspace" on Windows where this also appears) in the editor
   Expected: A message that the editor is read-only appears like when pressing any other key that would modify the content.
   Actual: No message is shown.
   Problem: When not having an actual message that an editor is read-only the user gets confused when tries to delete something and gets no message why nothing is happening.

[octref]

In the left side of diff view, pressing any character would show the popup but pressing delete doesn't. Here's what I'm referring to:

[alexdima]

@octref I cannot reproduce. Perhaps you have bound Delete to another command? The pop-up shows for me when pressing Delete or Backspace