As far as I am aware we use panels for the File app preview windows, e.g. sound previews.

Thanks Steve! Is there anything else you can think of?
+fukino regarding File app preview window since he is in MTV :)

That is the only one I am actively aware of. Unfortunately it's a tricky thing to look for just within our code base, and who knows how many apps out there that use it (even though it only works on Chrome OS now).

I know that there is a way to query extension/app API usage, + rdevlin.cronin@chromium.org who should know how or at least know who knows how.

https://cs.chromium.org/chromium/src/ui/file_manager/audio_player/js/background.js?q=panel+file:%5Esrc/ui/file_manager/+package:%5Echromium$&dr=C

Thanks. I'll discuss with fukino in Tokyo next week.

For other apps, yes, we should evaluate the impact of removal. Since app should be agnostic to the panel behavior, I hope we still can remove it, especially it's ignored on other platforms.

Here is a more thorough (but imperfect) search of our codebase. The audio preview app is the only non test instance that I found:

https://cs.chromium.org/search/?q=type%5B+%5D*%5B:%3D%5D%5B%5E%3D%5D.*panel+package:%5Echromium$+lang:%5Ejavascript$&m=100&type=cs

Yes, audio player is using panel and I don't know other similar apps.
Converting the audio player to a normal app should not be so hard.

FWIW, I do rather like the way it behaves now :)

I prefer it too, actually :)

Don't worry, we'll repeal and replace it. :-p

<triage> Setting an owner for this as it's being investigated!

Discussed with warx@ offline. I'll work on both removal and implementing new behavior.

+msw, since he's refactoring shelf code and I see panels come up a lot.

Is the plan still to keep code behind a flag for a while?

>I'll work on both removal and implementing new behavior.
Doh, I meant "He".

However, looks like warx@ got an assigned task for himself, so I'll hold until he confirms with abodenha@.

> Is the plan still to keep code behind a flag for a while?
Plan is to replace it with similar behavior & simpler impl, behind a flag.

You don't have to port to mus+ash, or is it causing headache now?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0775f98187d5d895b15b46212f55d30956aa946e

commit 0775f98187d5d895b15b46212f55d30956aa946e
Author: warx <warx@chromium.org>
Date: Thu May 18 04:27:18 2017

Remove WINDOW_TYPE_V1_PANEL AppWindow type

Changes:
Currently, panel window is only used as Chrome OS's audio player, which is WINDOW_TYPE_PANEL. WINDOW_TYPE_V1_PANEL is not a AppWindow type in use any more. Remove it in code base.

BUG= 691099 
TEST=covered by tests

Review-Url: https://codereview.chromium.org/2886293002
Cr-Commit-Position: refs/heads/master@{#472661}

[modify] https://crrev.com/0775f98187d5d895b15b46212f55d30956aa946e/chrome/browser/extensions/api/tabs/tabs_api.cc
[modify] https://crrev.com/0775f98187d5d895b15b46212f55d30956aa946e/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc
[modify] https://crrev.com/0775f98187d5d895b15b46212f55d30956aa946e/chrome/browser/ui/views/apps/chrome_native_app_window_views.cc
[modify] https://crrev.com/0775f98187d5d895b15b46212f55d30956aa946e/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.cc
[modify] https://crrev.com/0775f98187d5d895b15b46212f55d30956aa946e/extensions/browser/app_window/app_window.h
[modify] https://crrev.com/0775f98187d5d895b15b46212f55d30956aa946e/extensions/components/native_app_window/native_app_window_views.cc
[modify] https://crrev.com/0775f98187d5d895b15b46212f55d30956aa946e/extensions/components/native_app_window/native_app_window_views.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3cb8f0afe629acb8806b0d20ec00d539f7114406

commit 3cb8f0afe629acb8806b0d20ec00d539f7114406
Author: treib <treib@chromium.org>
Date: Thu May 18 08:28:08 2017

Revert of Remove WINDOW_TYPE_V1_PANEL AppWindow type (patchset #2 id:20001 of https://codereview.chromium.org/2886293002/ )

Reason for revert:
Breaks compile on ChromiumOS x86-generic due to unused-but-set variable saw_focus_key

Original issue's description:
> Remove WINDOW_TYPE_V1_PANEL AppWindow type
>
> Changes:
> Currently, panel window is only used as Chrome OS's audio player, which is WINDOW_TYPE_PANEL. WINDOW_TYPE_V1_PANEL is not a AppWindow type in use any more. Remove it in code base.
>
> BUG= 691099 
> TEST=covered by tests
>
> Review-Url: https://codereview.chromium.org/2886293002
> Cr-Commit-Position: refs/heads/master@{#472661}
> Committed: https://chromium.googlesource.com/chromium/src/+/0775f98187d5d895b15b46212f55d30956aa946e

TBR=benwells@chromium.org,oshima@chromium.org,warx@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG= 691099 

Review-Url: https://codereview.chromium.org/2891043002
Cr-Commit-Position: refs/heads/master@{#472740}

[modify] https://crrev.com/3cb8f0afe629acb8806b0d20ec00d539f7114406/chrome/browser/extensions/api/tabs/tabs_api.cc
[modify] https://crrev.com/3cb8f0afe629acb8806b0d20ec00d539f7114406/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc
[modify] https://crrev.com/3cb8f0afe629acb8806b0d20ec00d539f7114406/chrome/browser/ui/views/apps/chrome_native_app_window_views.cc
[modify] https://crrev.com/3cb8f0afe629acb8806b0d20ec00d539f7114406/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.cc
[modify] https://crrev.com/3cb8f0afe629acb8806b0d20ec00d539f7114406/extensions/browser/app_window/app_window.h
[modify] https://crrev.com/3cb8f0afe629acb8806b0d20ec00d539f7114406/extensions/components/native_app_window/native_app_window_views.cc
[modify] https://crrev.com/3cb8f0afe629acb8806b0d20ec00d539f7114406/extensions/components/native_app_window/native_app_window_views.h

Make sure you address this compile error when you reland:

chromeos-chrome-60.0.3104.0_alpha-r1: FAILED: obj/chrome/browser/extensions/extensions/tabs_api.o 
chromeos-chrome-60.0.3104.0_alpha-r1: i686-pc-linux-gnu-g++ -B/usr/x86_64-pc-linux-gnu/i686-pc-linux-gnu/binutils-bin/2.25.51-gold -MMD -MF obj/chrome/browser/extensions/extensions/tabs_api.o.d -DENABLE_HOTWORDING -DV8_DEPRECATION_WARNINGS -DUSE_UDEV -DUSE_ASH=1 -DUSE_AURA=1 -DUSE_NSS_CERTS=1 -DUSE_OZONE=1 -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD -DSAFE_BROWSING_DB_LOCAL -DOFFICIAL_BUILD -DGOOGLE_CHROME_BUILD -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -DNO_UNWIND_TABLES -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -DOS_CHROMEOS -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DUSE_EGL -DTOOLKIT_VIEWS=1 -DGOOGLE_PROTOBUF_NO_RTTI -DGOOGLE_PROTOBUF_NO_STATIC_INITIALIZER -DHAVE_PTHREAD -DV8_USE_EXTERNAL_STARTUP_DATA -DU_USING_ICU_NAMESPACE=0 -DU_ENABLE_DYLOAD=0 -DU_STATIC_IMPLEMENTATION -DICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_FILE -DUCHAR_TYPE=uint16_t -DSK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS -DSK_HAS_PNG_LIBRARY -DSK_HAS_WEBP_LIBRARY -DSK_HAS_JPEG_LIBRARY -DSK_SUPPORT_GPU=1 -DMESA_EGL_NO_X11_HEADERS -DLEVELDB_PLATFORM_CHROMIUM=1 -DI18N_ADDRESSINPUT_USE_BASICTYPES_OVERRIDE=1 -DI18N_ADDRESS_VALIDATION_DATA_URL=\"https://chromium-i18n.appspot.com/ssl-aggregate-address/\" -DFEATURE_ENABLE_VOICEMAIL -DEXPAT_RELATIVE_PATH -DGTEST_RELATIVE_PATH -DNO_SOUND_SYSTEM -DWEBRTC_CHROMIUM_BUILD -DWEBRTC_POSIX -DWEBRTC_LINUX -DCHROMEOS -DRLZ_NETWORK_IMPLEMENTATION_CHROME_NET -I../../../../../../../home/chrome-bot/chrome_root/src -Igen -I/build/x86-alex/usr/include/dbus-1.0 -I/build/x86-alex/usr/lib/dbus-1.0/include -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/libwebp -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/khronos -I../../../../../../../home/chrome-bot/chrome_root/src/gpu -Igen -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/protobuf/src -Igen/protoc_out -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/protobuf/src -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/ced/src -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/icu/source/common -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/icu/source/i18n -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/libwebm/source -I../../../../../../../home/chrome-bot/chrome_root/src/skia/config -I../../../../../../../home/chrome-bot/chrome_root/src/skia/ext -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/c -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/config -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/core -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/effects -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/encode -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/images -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/lazy -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/pathops -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/pdf -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/pipe -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/ports -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/utils -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/third_party/vulkan -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/include/gpu -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/src/gpu -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/skia/src/sksl -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/boringssl/src/include -I/build/x86-alex/usr/include/nss -I/build/x86-alex/usr/include/nspr -Igen -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/WebKit -Igen/third_party/WebKit -I../../../../../../../home/chrome-bot/chrome_root/src/v8/include -Igen/v8/include -Igen/components/metrics/proto -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/re2/src -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/mesa/src/include -Igen -Igen -Igen -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/cacheinvalidation/overrides -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/cacheinvalidation/src -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/leveldatabase -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/leveldatabase/src -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/leveldatabase/src/include -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/libaddressinput/src/cpp/include -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/libaddressinput/chromium/override -I../../../../../../../home/chrome-bot/chrome_root/src/third_party/webrtc_overrides -I../../../../../../../home/chrome-bot/chrome_root/src/third_party -fno-strict-aliasing -fno-unwind-tables -fno-asynchronous-unwind-tables -fPIC -pipe -m32 -msse2 -mfpmath=sse -mmmx -pthread -Wall -Werror -Wno-unused-local-typedefs -Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter -O2 -fno-ident -fdata-sections -ffunction-sections -fno-omit-frame-pointer -g2 -gsplit-dwarf --sysroot=../../../../../../../build/x86-alex -fvisibility=hidden -std=gnu++11 -Wno-narrowing -fno-rtti -fno-exceptions -fvisibility-inlines-hidden -march=i686 -pipe -march=i686 -pipe -pipe -march=atom -mtune=atom -mfpmath=sse -D__google_stl_debug_vector=1 -femit-struct-debug-reduced -c ../../../../../../../home/chrome-bot/chrome_root/src/chrome/browser/extensions/api/tabs/tabs_api.cc -o obj/chrome/browser/extensions/extensions/tabs_api.o
chromeos-chrome-60.0.3104.0_alpha-r1: ../../../../../../../home/chrome-bot/chrome_root/src/chrome/browser/extensions/api/tabs/tabs_api.cc: In member function 'virtual ExtensionFunction::ResponseAction extensions::WindowsCreateFunction::Run()':
chromeos-chrome-60.0.3104.0_alpha-r1: ../../../../../../../home/chrome-bot/chrome_root/src/chrome/browser/extensions/api/tabs/tabs_api.cc:484:8: error: variable 'saw_focus_key' set but not used [-Werror=unused-but-set-variable]
chromeos-chrome-60.0.3104.0_alpha-r1:    bool saw_focus_key = false;

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/64e280e83e00b3d5217595c55312a3251f682fdf

commit 64e280e83e00b3d5217595c55312a3251f682fdf
Author: warx <warx@chromium.org>
Date: Thu May 18 17:15:04 2017

reland: Remove WINDOW_TYPE_V1_PANEL AppWindow type

Changes:
Currently, panel window is only used as Chrome OS's audio player, which is WINDOW_TYPE_PANEL. WINDOW_TYPE_V1_PANEL is not a AppWindow type in use any more. Remove it in code base.

TBR=oshima@chromium.org, benwells@chromium.org
BUG= 691099 
TEST=covered by tests

Review-Url: https://codereview.chromium.org/2886293002
Cr-Commit-Position: refs/heads/master@{#472661}
Committed: https://chromium.googlesource.com/chromium/src/+/0775f98187d5d895b15b46212f55d30956aa946e

patch from issue 2886293002 at patchset 20001 (http://crrev.com/2886293002#ps20001)

Review-Url: https://codereview.chromium.org/2891963002
Cr-Commit-Position: refs/heads/master@{#472853}

[modify] https://crrev.com/64e280e83e00b3d5217595c55312a3251f682fdf/chrome/browser/extensions/api/tabs/tabs_api.cc
[modify] https://crrev.com/64e280e83e00b3d5217595c55312a3251f682fdf/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.cc
[modify] https://crrev.com/64e280e83e00b3d5217595c55312a3251f682fdf/chrome/browser/ui/views/apps/chrome_native_app_window_views.cc
[modify] https://crrev.com/64e280e83e00b3d5217595c55312a3251f682fdf/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.cc
[modify] https://crrev.com/64e280e83e00b3d5217595c55312a3251f682fdf/extensions/browser/app_window/app_window.h
[modify] https://crrev.com/64e280e83e00b3d5217595c55312a3251f682fdf/extensions/components/native_app_window/native_app_window_views.cc
[modify] https://crrev.com/64e280e83e00b3d5217595c55312a3251f682fdf/extensions/components/native_app_window/native_app_window_views.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e389a8213ee8167d6de0cfd55ad9c4a9ce9b7b14

commit e389a8213ee8167d6de0cfd55ad9c4a9ce9b7b14
Author: warx <warx@chromium.org>
Date: Thu May 18 17:53:18 2017

Remove AshPanelContents

Changes:
This is a follow up CL of crrev.com/2886293002. ash_panel_contents and launcher_favicon_loader is removed in this CL.

BUG= 691099 
TEST=covered by tests

Review-Url: https://codereview.chromium.org/2890983003
Cr-Commit-Position: refs/heads/master@{#472870}

[modify] https://crrev.com/e389a8213ee8167d6de0cfd55ad9c4a9ce9b7b14/chrome/browser/extensions/BUILD.gn
[delete] https://crrev.com/e7e125cf7719eaf113101afd815c067f0dad5798/chrome/browser/extensions/api/tabs/ash_panel_contents.cc
[delete] https://crrev.com/e7e125cf7719eaf113101afd815c067f0dad5798/chrome/browser/extensions/api/tabs/ash_panel_contents.h
[modify] https://crrev.com/e389a8213ee8167d6de0cfd55ad9c4a9ce9b7b14/chrome/browser/ui/BUILD.gn
[delete] https://crrev.com/e7e125cf7719eaf113101afd815c067f0dad5798/chrome/browser/ui/ash/launcher/launcher_favicon_loader.cc
[delete] https://crrev.com/e7e125cf7719eaf113101afd815c067f0dad5798/chrome/browser/ui/ash/launcher/launcher_favicon_loader.h
[delete] https://crrev.com/e7e125cf7719eaf113101afd815c067f0dad5798/chrome/browser/ui/ash/launcher/launcher_favicon_loader_browsertest.cc
[modify] https://crrev.com/e389a8213ee8167d6de0cfd55ad9c4a9ce9b7b14/chrome/test/BUILD.gn