First of all, yes this bug is still present. Second, the bug itself doesn't actually seem to depend on the window being maximized; that makes it more of a problem, but that pixel or two on the edge of the toolbar doesn't respond regardless of the window size.

With the hamburger menu that clickable area to the right of the icon is the right padding on the toolbarbutton, same as to the left of the back button as the nav-bar toolbar has no padding of it's own.

The bookmarks bar does have padding, so I tried updating the bookmarks toolbar chevron to have the full inline-end padding instead. This works, but I can't see how this is the expected behavior. No other toolbar buttons that I can find, outside of in the nav-bar, allow clicking outside of the hover area to trigger that button.

I've tried a few approaches to make this work in a generalized way for the PersonalToolbar, but keep running into dead ends with how the toolbarbutton.bookmark-item are structured with background on hover covering the entire button instead of just the image + label. Ideally we'd have those wrapped inside of the xul toolbarbutton element but that seem like a pretty far reaching change for fixing this issue?

This is preventing me from generally applying a right / left padding on the first / last element in the toolbar like is done on the nav-bar.

Going to try and find a more specific fix for when the PlacesChevron button is shown if possible.

I may have just stared at this too long, but I do think the only way to keep this consistent is to update the toolbarbutton class to something like:

<hbox class="toolbarbutton-wrapper">
          <image class="toolbarbutton-icon"></image>
          <label class="toolbarbutton-text" crop="end" flex="1"></label>
</hbox>

with the CSS hover background update to be on the .toolbarbutton-wrapper.

Unfortunately this breaks the styles (and maybe some JS?) for all other button in the toolbar, so I don't think it is a reasonable solution at this time.