2051903 - new windows: browser UI rendered slowly after opening if Nova design enabled

Status: RESOLVED DUPLICATE of bug 2056929

(Firefox :: General, defect, P2)

Description

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

Firefox 154.0a1 on Windows 11 with the Nova design enabled.

If a new private windows has been opened (e.g. with Ctrl/Cmd+Shift+P), the window is opened but there is hang of 500-800ms when the bookmarks sidebar and content are completely empty and the addressbar hangs below its expected position because it tried to slide in from the bottom when the hang started. This has been observed with a browser profile with many (thousands) of tabs in existing windows.

Comment 1

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

This affects all windows: non-private windows and also the 'About Firefox' window opened from the Help menu.

Comment 2

[BugBot [:suhaib / :marco]]

The Bugbug bot thinks this bug should belong to the 'Firefox::Sidebar' component, but is not confident enough to move the bug to that component.

Comment 3

[Sarah Clements [:sclements] | PTO until Sept 8th]

Sebastian, can you please provide profiles with and without nova performing the same actions? Can you confirm you're using the new sidebar with new bookmarks panel?

Comment 4

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

The behavior is not reproducible anymore. As far as I know some changes to the private browsing window landed recently.

Comment 5

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

The stuttering/hanging slide effect of the address bar is not reproducible anymore but opening new windows is still slow and noticeable because:

• the first private window opened has its tab bar background as gray area (Windows 11) until its rendered in purple. This can be slow (profile has many tabs open)
• menu Help and Report > About Nightly opens the window but it remains blank for one second until the content is rendered. Its javascript already runs before the render because the update check is complete when the content is shown.

Comment 6

[Stephen Thompson [:sthompson]]

Hi :aryx, thank you for reopening. There are a couple of somewhat related reports about perceived slowness in windows with many tabs, e.g. bug 2058260. If you can reproduce relatively consistently, could you please record a performance profile while opening a new private window and/or the About Nightly window?

Comment 7

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

First private window opened: https://share.firefox.dev/4fNogl4
About Nightly window opened & closed: https://share.firefox.dev/4ftt28o

Comment 8

[Stephen Thompson [:sthompson]]

I did some analysis with profiler-cli and captured a private window open on macOS 154 Beta with Nova enabled.

Your private window opening profile: CustomizableUI takes over 1 second to add toolbarbuttons, and there's over 1 second of layout thrashing later that overlaps with @testpilot-contains onFocusChanged event handler

Your About Nightly opening profile: 2.7 seconds to load the loading.svg, major GC event during the whole load, and @testpilot-contains onFocusChanged overlaps layout thrashing for 2 seconds. Claude Code also highlighted that 90% of samples in the period were in BrowsingContext::PreOrderWalkVoid going through all of the browser contexts, though I'm not sure what to make of that. I don't know if that's expected behavior and it's just unlucky that it was overlapping a window open.

My private window opening profile: everything completes in a few hundred milliseconds. Nothing really stands out to me. I have almost 5,000 tabs in 3 windows, although I recently restarted and so only a few tabs are actively loaded. I have 4 extensions.

I think it would be a good idea to follow up on Sarah's comment 3 to compare two profiles with Nova enabled and disabled. There is some significant style work happening in your profiles and those could be more taxing under Nova, but most of the expensive things I saw in your profiles didn't seem to me like they would be different under Nova.

Comment 9

[Markus Stange [:mstange]]

(In reply to Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout) from comment #7)

First private window opened: https://share.firefox.dev/4fNogl4
About Nightly window opened & closed: https://share.firefox.dev/4ftt28o

Sebastian, can you capture both Nova and non-Nova versions of these profiles? Then we can compare exactly which times changed.

Comment 10

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

Performance profiles with Firefox 155.0a1 20260804043244 - newer versions cannot be used with this profile, see bug 2056929 comment 9.

with Nova:
first private browsing window opened: https://share.firefox.dev/4fQ0ncn
About Firefox window opened: https://share.firefox.dev/4w4gZ6f

without Nova
first private browsing window opened: https://share.firefox.dev/4hyY8Nl
About Firefox window opened: https://share.firefox.dev/3SahvSp

Comment 11

[Andrew Creskey [:acreskey]]

In bug 2059857 Markus captured the cost of switching window activeness, which we see in both the new browsing window creation scenario and the About Firefox window being opened.

An independent issue that I see here is the time spent in handleNewBrowserWindow which is 1388ms with Nova, and 81ms without Nova.

Nova: https://share.firefox.dev/4hTHWWM
Non-Nova: https://share.firefox.dev/4zl8KWz

Comment 12

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

Firefox 156.0a1 build ID 20260816083833 - the issues is gone.