I've narrowed the behavior a bit. The button does not need to be clicked to trigger the behavior; simply closing the tab with ^W _while(iff) the button is visible_ will trigger it. After the result-view is re-opened (and the button produced), hovering will not change the background.

I've added print-outs (or stepped through) from OmniboxTabSwitchButton::StateChanged(), and MdTextButton::SetBgColorOverride() down to the bottom of Compositor::SchedulePaint. I don't see any difference between before the close and after. I've also attempted random shots in the dark, such as calling SetVisible() in a few places.

What's weird is that the ink-drop effect works, when I click on it, so something in the drawing is working.

It still feels like something at the bottom is ignoring the background change, but the ink-drop behavior implies that View or MdTextButton has the power to overcome that. Regardless, it looks like this is well outside my area of expertise, so I'd appreciate any suggestions.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9f7342bf40c9ad522e905d446218d5baf37c73ab

commit 9f7342bf40c9ad522e905d446218d5baf37c73ab
Author: Kevin Bailey <krb@chromium.org>
Date: Tue Apr 03 21:20:30 2018

[omnibox] Recreating tab switch button on appropriate set match

The tab switch button won't show background changes after being hidden
and re-shown. This is to work-around that, by recreating it on each set
match that needs it. (Creating it is needed if the match type is a tab
switch and the tab switch button behavior was enabled.) This change
also restores the strategy of not creating a tab switch button unless
and until needed.

Bug:  825372 
Change-Id: I0b808969ee5e9b91e3ca292fa929447da148d75c
Reviewed-on: https://chromium-review.googlesource.com/992356
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547834}
[modify] https://crrev.com/9f7342bf40c9ad522e905d446218d5baf37c73ab/chrome/browser/ui/views/omnibox/omnibox_result_view.cc
[modify] https://crrev.com/9f7342bf40c9ad522e905d446218d5baf37c73ab/chrome/browser/ui/views/omnibox/omnibox_tab_switch_button.cc

Tried checking the issue on chrome version 67.0.3387.0 and 67.0.3388.0 using Ubuntu 14.04 by following the below mentioned steps.
1. Enabled tab switch button feature in chrome://flags
2. Opened example.com
3. Opened another tab, typed example.com in Omnibox and clicked on tab switch button.
4. opened another tab and typed example.com.
We observed similar behaviour on both the versions with respect to background color. attaching the screen shots of the same.

@Kevin Bailey: Could you please have a look at those screen shots and let us know if anything missed. Please help us in verifying the fix.

Thanks!

Deleted: 825372-3387.png 264 KB

	825372-3387.png 264 KB View Download

Deleted: 825372-3388.png 280 KB

	825372-3388.png 280 KB View Download

Hi Vamshi, I don't see the tab switch button at all in your screenshots. But don't worry about it:

I don't if you noticed that I'm both the reporter and owner. As I mentioned in the first comment, I wrote the bug so I wouldn't forget it over a break, and to ask for help. I've worked around the bug with the CL above.

I'm going to close this, verified, but others are welcome to look into the strange behavior with this button type, or provide suggestions.