Unsaved changes lost after "**Save As...**" operation

[per1234]

Describe the bug

The IDE give the users the option of two save behaviors, configurable via File > Preferences > ☐ Auto save:

• 🗹 Auto save - changes to the sketch are saved to disk ~immediately
• ☐ Auto save - changes to the sketch are stored in memory and saved to disk only when the user explicitly instructs the IDE to do so (e.g., File > Save)

🐛 Unsaved changes are not written to disk when the user does a "Save As..." operation.

To Reproduce

1. Select File > Preferences from the Arduino IDE menus.
2. Uncheck the box next to "☐ Auto save"
3. Click the OK button.
4. Select File > New from the Arduino IDE menus.
   ℹ️ The bug is not limited to new sketches. It also occurs with sketches that have been previously saved to a permanent location on the disk.
5. Make some change to the sketch.
   ❗ Do not save.
6. Select File > Save As... from the Arduino IDE menus.
7. Click the Save button.

🐛 The changes you made are gone.

Expected behavior

Unsaved changes are saved to the target sketch during a "Save As..." operation.

Desktop

• OS: Windows 10, Ubuntu 20.04
• Version: 2.0.0-rc4-snapshot-70f1c5f
  Date: 2022-03-09T10:47:19.505Z
  CLI Version: 0.21.0 [10107d24]

Additional context

It is not a matter of the changes being saved to the wrong copy of the sketch. If you start from a previously saved sketch, the unsaved changes will not be in that one either.

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This was originally reported at #38, but since the primary focus of that issue has since been resolved I decided it would be best to close #38 and track the remaining part in a dedicated issue.

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Also reported at #829 (comment)

[alranel]

I confirm this behavior as of 2.0.0-rc6.
It can be reproduced also by creating a new tab (file), writing something in it and then performing a save: the content of the new tab is lost.