Availability of source selection in create merge request button depends on correct timing

Summary

The Create merge request dropdown uses the master branch when the Source field is not filled out. In certain situations and with the correct timing, an empty field that would otherwise default to master can evaluate to Source not available.

Source is not available Source is available
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Steps to reproduce

  1. Open Create merge request dropdown
  2. Delete every content in Source field
  3. Type in aa
  4. Delete one a
  5. wait until you see Checking for source availability.., then directly delete the second a
  6. The status now is Source is not available
  7. Press the backspace key
  8. The status now is Source is available

What is the current bug behavior?

Empty Source field defaults to master and evaluates to Source is not available

What is the expected correct behavior?

Empty Source field defaults to master and evaluates to Source is available