What were you doing?

1. Open Debugger on https://debugger-pretty-print-events-1532240.glitch.me/
2. Enable mousemove in Event Listener Breakpoints
3. Close Debugger
4. Open Debugger and move mouse over page

What happened?

https://perfht.ml/362xmWg

1. Paused overlay shows instantly, Call stack shows a function reference
2. No file is shown
3. Debugger seems idle for 10s
4. 10s later the file shows, triggering 3 expensive reflows: https://perfht.ml/2BIsTKc
5. After that, several expensive double-reflows from bug 1591000: https://perfht.ml/2JiMJjo

What should have happened?

At least: Show that file is loading earlier.
Even better: Don't make users wait 10sec

Anything else we should know?

parser-worker.js takes

• Mostly extractSymbols
• 1.5sec for a structured clone to post the data back: https://perfht.ml/2W8ONzQ

Sending data then causes 1.5sec of structured clone reading. https://perfht.ml/33SM6oG

Is there anything obviously wrong with how the UI waits for parsing?

No, I think it's possibly just a general "large file" problem:

https://searchfox.org/mozilla-central/source/devtools/client/debugger/src/actions/sources/select.js#147-162

We add the tab based on source, which we remap once we mapLocation, which relies on sourcemaps.

We should create a "large files" meta where we track these and put them into a dbg-7* sprint.

David, what about:

At least: Show that file is loading earlier.

At least having UI state would help a lot.

We add the tab based on source, which we remap once we mapLocation, which relies on sourcemaps.

How does sourcemaps affect this STR?

Some things I've found interesting in https://perfht.ml/362xmWg at first glance:

• 421ms spent in a getter at get resource://devtools/client/debugger/dist/parser-worker.js:653
• 291ms spent in another getter at get typeresource://devtools/client/debugger/dist/parser-worker.js:17247:10`
• 213ms spent in another getter at get node resource://devtools/client/debugger/dist/parser-worker.js:17211:10

The first one is probably added by Babel/Webpack, while the last ones come from simple-path.js.

At least: Show that file is loading earlier.

https://phabricator.services.mozilla.com/D50255 should help with that.

This bug was caused by https://phabricator.services.mozilla.com/D50564

We can't reproduce this anymore.