When do toolbar buttons get focus in order to actually get keyboard events? I found this comment in custom_button.h:

// A button with custom rendering. The base of ImageButton and LabelButton.
// Note that this type of button is not focusable by default and will not be
// part of the focus chain.  Call SetFocusBehavior(FocusBehavior::ALWAYS) to
// make it part of the focus chain.

Pressing <alt> + <shift> + 't' will focus the Toolbar buttons.

This appears to be working now. I'm seeing the ripple feedback in MD mode on pressing the spacebar.

you're seeing a ripple before release the space key?

It's odd. I see the ripples on the button to the right of the omnibar, but not the refresh button... so it's not consistent or universal to all of the buttons.

Upon further examination, the ripples do appear on release.. if the timing is right. Also, the ripples are much larger for the back/forward/refresh buttons than the app/extension/menu buttons on the right.

The navigation button actions are triggered on space-release whereas the extension button actions are triggered on space-down.  I believe the ACTION_TRIGGERED ripple animation is triggered when the button action is triggered, thus you will see the ripple on space-down for extension buttons but not for the navigation buttons.  This issue is tracking the work to show the ripple on space-down for space-up triggered buttons.

Ok, that's why I didn't initially notice it. I assumed (yeah, I know ;-), that the extension buttons shared the same implementation as the navigation buttons.

To make this work then, the ripple trigger will need to be tied to a specific action applied to the button and not the resulting event trigger.

with regards to the discrepancy between nav and extension buttons, see also
https://codereview.chromium.org/1886943002
https://codereview.chromium.org/1947533003

Note in particular my comment "We'd also need to update browser
action buttons to key up, but that can be a TODO."

> This issue is tracking the work to show the ripple on space-down for space-up triggered buttons.

correct

Marking to started for kylixrd@.

Review: https://codereview.chromium.org/1977793006/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bc4847d61fd0da6cd297f60311d9d7c58f391c75

commit bc4847d61fd0da6cd297f60311d9d7c58f391c75
Author: kylixrd <kylixrd@chromium.org>
Date: Mon May 16 19:50:40 2016

In MD mode, pressing spacebar on toolbar buttons triggers the ripple effect. It will remain in that state unti the key is released

BUG= 608996 

Review-Url: https://codereview.chromium.org/1977793006
Cr-Commit-Position: refs/heads/master@{#393907}

[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/animation/button_ink_drop_delegate.cc
[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/animation/button_ink_drop_delegate.h
[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/animation/ink_drop_delegate.h
[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/animation/test/test_ink_drop_delegate.cc
[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/animation/test/test_ink_drop_delegate.h
[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/controls/button/custom_button.cc
[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/controls/button/custom_button_unittest.cc
[modify] https://crrev.com/bc4847d61fd0da6cd297f60311d9d7c58f391c75/ui/views/controls/button/menu_button_unittest.cc