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Ah, thanks, I missed there was already an issue.
The patch's logic is as follows: when pressing tab anywhere in the line, if the word to complete is empty (which might be for any number of reasons, like the cursor is after another space or a non-word character), then suggest as completion a string of N spaces, where N is a number from 1 to 4 so that the cursor ends up being aligned to a multiple of 4. You have to press backspace several times to remove all these spaces.
Indeed, it could also suggest a single '\t' character, which would be simpler and revert exactly to the old behavior (including a single backspace to remove the whole tab). I didn't do it only because it would typically indent to 8 characters instead of 4. If people think it is anyway a better idea, I can submit a second (simpler) patch doing that.