I found an empty color box would appear at the end of a page if the space is not enough for a whole box. Here is an example:
#set quote(block: true)
// show quote: set pad(left: 2em)
#show quote: set block(
fill: luma(230),
width: 100%,
inset: 5pt,
radius: 5pt,
)
#lorem(630)
#quote[
This is a quote.
]
The sticky parameter worked but it also prevented the box from being split across pages. Sometimes a flexible box will be better. I just want to avoid the weird blank line in the box.