Created attachment 215487 [details]
HTML file highlighting the problem
Setting a forced break via Web Inspector in an element's style will not change the layout to reflect the break unless a forced layout is triggered (e.g. by resizing the browser window).
Changing a rule however, will immediately change the layout.
1. Open attached file
2. Open WebInspector and select first <p> in the <article>
3. Add in its style "-webkit-region-break-after: always"
EXPECTED: Second and third paragraphs flow in the second box
ACTUAL: Second and third paragraphs will not flow in the second box unless you resize the browser window
4. Add "-webkit-region-break-after: always" in the empty "article p" rule
EXPECTED & ACTUAL: Layout changes immediately
I am not sure what you mean by "add in its style". Do you mean, setting the style="" attribute on the element, or editing a CSS rule in the Elements tab?
By "add in its style" I meant "edit the style of the element in the WebInspector". This effectively means editing the "style" attribute of the element.
CSS Regions were removed in Bug 174978.