For `page_sidebar` and `page_navbar`, consider changing default for `fillable` to `False`

[wch]

For Express mode we had set the default page type to page_fillable(), but changed it back to page_fixed() for reasons explained in #886.

In Express mode, when a sidebar is added, it uses page_sidebar(), which defaults to fillable=True and then runs into the same problem. That means that from the user's perspective, simply adding a sidebar makes a big change in how the main content of the page is laid out, and can lead to surprising errors. For example:

Then the user needs to know to use ui.page_opts(fillable=False) to fix it.

Especially in cases where there is a lot of content on the page, I think it would be very surprising for a user that simply adding a sidebar would result in this change; and then in order to understand the fix, they would need to understand the fillable concept, and that the sidebar changes the default behavior of it.

[jcheng5]

I'm inclined to agree, though I think we should discuss in person with @cpsievert and @gadenbuie as I know they put a lot of thought into it.

[wch]

FWIW, I ran into this while working on example apps for the docs, so I don't think it would be an unusual problem to users to encounter.

[cpsievert]

I can get on board with fillable=False for page_sidebar() -- for consistency, we'll also want that for page_navbar() (and possibly doing the same for bslib?).