I suspect that the titlebar repainting on mouse waggles is also unintended, or at least avoidable.

Can you land your changes that add tracing events around the custom titlebar tracing?

Is it possible to land equivalent events for normal titlebar painting when the custom titlebar is disabled? I assume not (since Windows is doing that), but ???

I retested today and it looks like simply scrolling does NOT cause the titlebar to repaint, and neither does waggling the mouse over the non-interactive titlebar background. However, waggling the mouse over the active tab does cause it to repaint (even though nothing is changing).

That means this might not explain the perf test regressions though, since I think they're using synthetic mouse events to scroll the page.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8e297c1c23e884e7c49c6913b161b4c5361179f1

commit 8e297c1c23e884e7c49c6913b161b4c5361179f1
Author: Bret Sepulveda <bsep@chromium.org>
Date: Fri Apr 06 01:46:59 2018

Add trace events to window frame code, to measure performance.

Bug: 829137
Change-Id: I1bcce9e0a61d0528318d9c58814fba3de9377434
Reviewed-on: https://chromium-review.googlesource.com/998997
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548635}
[modify] https://crrev.com/8e297c1c23e884e7c49c6913b161b4c5361179f1/chrome/browser/ui/views/frame/glass_browser_frame_view.cc
[modify] https://crrev.com/8e297c1c23e884e7c49c6913b161b4c5361179f1/chrome/browser/ui/views/frame/opaque_browser_frame_view.cc
[modify] https://crrev.com/8e297c1c23e884e7c49c6913b161b4c5361179f1/chrome/browser/ui/views/frame/opaque_browser_frame_view_layout.cc

This is the first time I've used chrome://tracing, but it looks to me like there's a fairly fixed minimum of PaintTitlebar calls on the initial display of a tab and switching back to the tab recording the trace - on the order of 60.

The vast majority of the paint calls are due to mousing over the title bar. If I start recording, use <cntrl> T to open a new tab, <cntrl> <shift> Tab to get to the about:tracing tab, and stop recording, I see 2 calls to LayoutTitleBar and 8 calls to paint. Using the mouse to switch to the about:recording tab causes ~45 calls to paint, which is consistent with what Bret described about waggling the mouse over the active tab.

Moving the mouse over the app menu seems to generate 2 or 3 paints of the title bar, even though it's not part of the title bar.

It looks like opening or switching tabs generates about 4 paints, so depending on what the performance tests are doing, I suppose it could show up.

I think the critical question is whether the amount of repainting is varying depending on whether we have the custom title bar enabled - assuming that we can determine that.

On the other hand, maybe that doesn't matter. If we own the title bar and if we are repainting it when nothing has changed then we should stop doing that.

Maybe we have some logic that triggers repaints on every mouse move event instead of on every mouse move event that changes state. That would end up being a small but important performance regression.

Turning off the custom title bar via the flag reduces the number of calls to OnPaint in both scenarios. <Cntrl> T <Cntrl> <Shift> Tab generates 7 calls to OnPaint and using the mouse to switch back to the previous tab generates ~31 calls to Paint instead of ~45. The former doesn't seem all that significant but the latter does, though 31 calls to OnPaint is still a surprising number.

Specifically, mousing over tabs generates a lot of OnPaint calls. Mousing elsewhere in the title bar doesn't.

I ran the perf test in https://bugs.chromium.org/p/chromium/issues/detail?id=825814 locally, with GlassBrowserFrameView tracing added to the benchmark tracing, and GlassBrowserFrameView::OnPaint is only called 5 times, so I doubt that perf test regression is due to the window frame getting painted more.  

***Mass UI Triage*** As per dev comments adding related labels for further triage.Thanks!