1304678 - getComputedStyle return wrong style in print mode

Status: RESOLVED INVALID

(Core :: CSS Parsing and Computation, defect)

Description

[guillaume.baty]

Attachment: bug_get_computed_style_ff.html

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0
Build ID: 20160921030221

Steps to reproduce:

Create a basic html page with 2 styles:
   - one for @media screen
   - one for @media print

Watch events 
  - window.onbeforeprint
  - window.onafterprint

Get computed style each time these events are triggered (by using "print" or "print preview". Note: emulate media print do not trigger these events)

See attached example



Actual results:

getComputedStyle always return "@media screen" style


Expected results:

style returned by getComputedStyle should correspond to "@media print" style at least in one of the two events: onbeforeprint, onafterprint.

Maybe I am wrong for when print style should be applied or how to manage this case (get "@media print" computed style) ... 

Thank you for you answer

Comment 1

[Tooru Fujisawa [:arai]]

I don't see anything related to JS engine in this bug.

anyway, those events are not related to actual printing, but they're called before and after.
returning "@media screen" style should be correct.

Comment 2

[Xidorn Quan [:xidorn] UTC+10]

I agree with :arai that this seems to be the expected behavior. Those events are triggered before and after the printing, and the "@media print" should only be effective between them, so none of them should have computed value from "@media print".

I'm going to close this bug as INVALID. However I can understand that people may want to run script based on the new style for printing. That is probably something the spec should add. But given the current spec, I believe Firefox's behavior is correct.

You may want to raise an issue to the HTML spec here: https://github.com/whatwg/html/issues