I think I cannot reproduce,
is it reproduceable with single Monitor also?
By Sign In, are you referring to the modal Connection Information dialog you get when a connection's password is not saved?
You need to be more precise in describing your test case. I thought perhaps you might be referring a scenario where...
1. The connection's password is not saved.
2. You have some code editor tabs automatically re-opening from a prior SQL Developer session.
3. The Tools > Preferences > Code Editor > Link Stored Procedures to Files checkbox is ticked.
4. The Connection Information dialog pops up automatically to prompt for the password.
In my testing, however, using 17.3.1 (and in a single monitor environment), the Connection Information dialog only opens if I click on the connection node in the navigator tree. Also, when I click on other windows, returning to SQL Developer always leaves Connection Information in front, never behind the main application window.
So Hendrik Schmidt's comment regarding reproducibility in a single vs multi-monitor environment is definitely relevant. There have been other issues reported against multiple monitor environments. For example: Using Multiple Monitors
Starting a second instance of SQL Developer and killing the previous SQL Developer instance is the only remedy I could find so far.
Starting a second instance creates another copy of system_cache (system_cache_1) in your user settings under the
SQL Developer\system<version> folder. So an alternate workaround would be to nuke the system_cache folder.
The only option is to kill the SQL Developer. But all subsequent starts will have the Sign In window hidden.
Just use alt-tab to switch between windows.