Run the browser with --force-ui-direction=rtl .  Install a theme such as https://chrome.google.com/webstore/detail/dots/gliedaffibdnbhbiaolgkdhhfbjgmhgi , where it's easy to see when things are misaligned.  Resize the window back and forth from the right edge.  Note how the tab and NTB backgrounds get misaligned; they seem to always be one resize event behind, so if you move the mouse rapidly and suddenly stop you can get arbitrarily large misalignment.

Not sure why this only seems to be wrong in RTL.  Wondering if it's something like, when we get the resize event we tell the tabstrip to update its background coordinate, and the tabstrip tries to calculate the mirrored X, but the width of the window hasn't been updated yet, so we use the old, out-of-date width and get the wrong coordinate as a result.

CCing some newer UI folks in case one of them wants to jump on this :)