(1) The easiest way to reproduce:
Type five white spaces in a caption - the Submit button becomes active and the empty caption can be submitted
- In add image caption type letters for a valid input - 5 characters or more, e.g. testing. The Submit button becomes active.
- Then add some trailing white spaces.
- After entering white spaces, delete the valid input - delete testing word - the Submit button remains active and the caption will be published as it is - with white spaces, i.e. it's blank.