Hello Tapani
I've reproduced the issue with resizing of opera on my machine (Intel® UHD Graphics 620) with Ubuntu 18.04, 5.1.2 version of the Kernel and 1.20.1 version of Xorg.

I've bisected the issue

2b2b449dd146c43860c71eb05f0f02ee2ad69854 is the first bad commit
commit 2b2b449dd146c43860c71eb05f0f02ee2ad69854
Author: Rafael Antognolli <rafael.antognolli@intel.com>
Date: Thu Feb 21 17:51:26 2019 -0800

iris: Enable auxiliary buffer support again

Now that we are properly resolving buffers before giving them to the
window system, let's enable aux support again.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

:040000 040000 57210508613c4dac2e8f8ce9a4b5c191d7c3ebe1 813a5a1ad6c56e0f0eae0364f87ba7006fa0e4d0 M	src

Thanks Paul for bisecting the issue! I've tried to add some resolve/flush to happen (based on old gitlab MR 526, version 3) but it did not fix the case, I might be doing it wrong though. Will continue investigation, we are likely missing a resolve somewhere.

There are new related regressions, now all the windows show artifacts independent of resizing. However, this commit fixes it and also with it there are no artifacts during resize:

https://gitlab.freedesktop.org/mesa/mesa/merge_requests/1843

Hello Tapani
I've also tested the issue with applied patch - looks good for both browsers.

--- 8< ---
commit f4d9169204e1b83f87779a350dd23501a085b36c
Author: Tapani Pälli <tapani.palli@intel.com>
Date:   Mon Sep 2 13:02:33 2019 +0300

    iris: disable aux on first get_param if not created with aux
    
    This moves the fix from commit 361f3d19f1f to happen in get_param
    (used now instead of get_handle by st/dri). This fixes artifacts
    seen with Xorg and CCS_E.
    
    Fixes: fc12fd05f56 "iris: Implement pipe_screen::resource_get_param"
    Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
    Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>