class list

Interesting.

Is it only a problem with a certain theme (do you see the same thing with Twenty Thirteen for example)?

Here’s my theory: a number of themes – yours included, perhaps – are prone to injecting lots of often unnecessary additional paragraph and break tags.

Themes may do this to try and achieve consistent formatting but the problem is they are applying formatting functions like wpautop() outwith the normal sequence for these things, which ultimately can be a bit of a pain.

(That would explain why you can’t replicate with a default theme and why it is so infrequently reported.)

So, basically, things look a bit odd when the widget initially loads because extra breaks are being introduced. Once you nav to another month it is refreshed directly by ajax, stopping your theme from interfering, so it looks as it should at that point.

I have no particular experience of your theme but I would recommend reaching out to the author and asking how this can be mitigated – I know some themes have a pseudo-shortcode like [noformat] that you can use to wrap up areas that shouldn’t be impacted in this way, but this obviously varies on a theme by theme basis.

(Also, note that this is just a theory! Experience tells me it is likely to be your theme which is “at fault”, particularly given the information in your last reply, but it’s not impossible that some other plugin could be behind this, too.)

I hope that helps!