STR;

1. Open https://www.irs.gov/pub/irs-pdf/f1040.pdf
2. Click the printer icon in the pdf viewer
3. Open the browser toolbox

You will see a blank window

Thanks for the report!

I can't reproduce the problem on my machine Win 10, Nightly 82.
Hiroyuki, what OS/Firefox version are you running?

Do you see any exception e.g. in the Browser Console?

Honza

I can also reproduce. Windows 10, latest Nightly.
Multiple exceptions in the browser console:

Failed to sendQuery in DevToolsFrameParent DevToolsFrameParent:instantiate-already-available DevToolsFrameParent.jsm:200:15
    sendQuery resource://devtools/server/connectors/js-window-actor/DevToolsFrameParent.jsm:200
    instantiateTarget resource://devtools/server/connectors/js-window-actor/DevToolsFrameParent.jsm:63
    createTargets resource://devtools/server/actors/watcher/target-helpers/frame-helper.js:39
    watchTargets resource://devtools/server/actors/watcher.js:137
    handler resource://devtools/shared/protocol/Actor.js:166
    onPacket resource://devtools/server/devtools-server-connection.js:379
    _onJSONObjectReady resource://devtools/shared/transport/transport.js:464
    makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103
    makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103

Error: Mismatch between DevToolsFrameParent and DevToolsFrameChild  expected browsing context with browserId undefined, but got 32212254721 DevToolsFrameParent.jsm:201:15
    sendQuery resource://devtools/server/connectors/js-window-actor/DevToolsFrameParent.jsm:201
    instantiateTarget resource://devtools/server/connectors/js-window-actor/DevToolsFrameParent.jsm:63
    createTargets resource://devtools/server/actors/watcher/target-helpers/frame-helper.js:39
    watchTargets resource://devtools/server/actors/watcher.js:137
    handler resource://devtools/shared/protocol/Actor.js:166
    onPacket resource://devtools/server/devtools-server-connection.js:379
    _onJSONObjectReady resource://devtools/shared/transport/transport.js:464
    makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103
    makeInfallible resource://devtools/shared/ThreadSafeDevToolsUtils.js:103

Error while calling actor 'watcher's method 'watchTargets' Mismatch between DevToolsFrameParent and DevToolsFrameChild  expected browsing context with browserId undefined, but got 32212254721 Actor.js:95:13
    writeError resource://devtools/shared/protocol/Actor.js:95
    handler resource://devtools/shared/protocol/Actor.js:199

receiveMessage@resource://devtools/server/connectors/js-window-actor/DevToolsFrameChild.jsm:337:15
JSActor query*sendQuery@resource://devtools/server/connectors/js-window-actor/DevToolsFrameParent.jsm:197:31
instantiateTarget@resource://devtools/server/connectors/js-window-actor/DevToolsFrameParent.jsm:63:17
createTargets@resource://devtools/server/actors/watcher/target-helpers/frame-helper.js:39:8
watchTargets@resource://devtools/server/actors/watcher.js:137:30
handler@resource://devtools/shared/protocol/Actor.js:166:37
onPacket@resource://devtools/server/devtools-server-connection.js:379:58
_onJSONObjectReady/<@resource://devtools/shared/transport/transport.js:464:22
exports.makeInfallible/<@resource://devtools/shared/ThreadSafeDevToolsUtils.js:103:22
exports.makeInfallible/<@resource://devtools/shared/ThreadSafeDevToolsUtils.js:103:22
Actor.js:100:15
    writeError resource://devtools/shared/protocol/Actor.js:100
    handler resource://devtools/shared/protocol/Actor.js:199

Alex, can you reproduce this?

Honza

I'm reproducing this, let me have a look

It looks like this is what causing the issue: https://searchfox.org/mozilla-central/rev/ce21a13035623c1d349980057d09000e70669802/devtools/server/connectors/js-window-actor/DevToolsFrameChild.jsm#34-41

One of the document in the page has docShell.hasLoadedNonBlankURI set to false, which makes us think it's an about:blank page, that we shouldn't handle, but is not detected from https://searchfox.org/mozilla-central/rev/ce21a13035623c1d349980057d09000e70669802/devtools/server/actors/watcher/target-helpers/frame-helper.js#247-253 , which ends up causing the issue in DevToolsFrameChild.

Setting as P2 since PDF printing is an important feature.
Thanks Nicolas for jumping in and helping!

Honza

PDF print preview are using about:blank documents, which are ignored
by DevTools in order to save resources.
We avoid creating targets for these, very often, transcient documents.
I tried to find ways to distinguish typical temporary about:blank documents
from this print preview one.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=79e19c96332c80ab01aaf8b7d6bf93487b7e1e26

https://treeherder.mozilla.org/#/jobs?repo=try&revision=45a151f24ceaf41b8d8efc1690107ffb420dea10

https://hg.mozilla.org/mozilla-central/rev/bbf6a9ee320a

Reproduced the issue on the affected Nightly build from 2020-08-27 on Windows 10.
Verified-fixed on the latest Nightly 83.0a1 (2020-09-22) (64-bit) and Beta 82.0b2 on Windows 10, MacOS 10.15 and Ubuntu 16.04. The browser toolbox will be opened and functional when a pdf document is in print preview.