I don't think we support changing combobox contents while they're open.
Do you have some use case where this happens or matters?
I don't see how it's related to  issue 771878 .

This isn't something I'd work on, nor should we implement without a good reason.
If we did change contents while open, how does that affect user interaction?
(ie. if you're about to click an entry and they shift/change, that's frustrating)
I would resolve this WontFix, but maybe you care to explain a bit more.

We need it for issue 753806. When a user click on the eye icon, the passwords in the combobox become masked/revealed. The combobox may be closed, but the selected item changes appearance. Does it make sense? 

I have call repaint here https://chromium-review.googlesource.com/c/chromium/src/+/723319/5/chrome/browser/ui/views/passwords/manage_passwords_bubble_view.cc#597 

Ah, you mean what's shown when the combobox is closed; sorry for misunderstanding.
Yeah, I suppose Combobox::ModelChanged() could call SchedulePaint() internally.

Will you have a chance to fix it?

This seems simple enough to include in your existing patch set, I'll review.

I am going to merge https://chromium-review.googlesource.com/c/chromium/src/+/723319 to M-63. To keep things safer, I will do in a separate CL that affects on all comboboxes.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/053f61ae949484a98b834541ffd51307ca678713

commit 053f61ae949484a98b834541ffd51307ca678713
Author: Maxim Kolosovskiy <kolos@chromium.org>
Date: Fri Nov 03 16:39:36 2017

[Views][Combobox] Schedule re-paint once a combobox model has changed

Bug:  775496 
Change-Id: I0a1a6884f57cfa3a587a0b02da6cadb437a03fb8
Reviewed-on: https://chromium-review.googlesource.com/727886
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513801}
[modify] https://crrev.com/053f61ae949484a98b834541ffd51307ca678713/chrome/browser/ui/views/passwords/manage_passwords_bubble_view.cc
[modify] https://crrev.com/053f61ae949484a98b834541ffd51307ca678713/ui/views/controls/combobox/combobox.cc