Affected versions

• Firefox 98.0b9
• Latest Nightly 99.0a1

Unaffected versions

• Firefox 97.0.1

Affected platforms

• macOS 11.6

Steps to reproduce

1. Open a webpage that has the scrollbar enabled (you can scroll in the page)
2. More the cursor of the mouse to the right side of Firefox
3. Click the scroll wheel to enable the autoscroll icon

Expected result

• The autoscroll icon is displayed and user can use autoscroll.

Actual result

• Autoscroll icon is not displayed but user can use autoscroll

Regression range

• First bad: de3e434c
• Last good: 98e35646
• Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=98e35646ac5a93de78ce61f72dff986adc268533&tochange=de3e434c51510837285c623bc417e23ee5b1000c
• Potential regressor: Bug 1741830 - Menu is too short when opened from the top of a screen located under a bigger screen

Additional notes

• I also tried with a Windows Dell Laptop which is not a hiDPI one and I could not reproduce this issue using that config.
• I could only reproduce this issue having two extra monitors connected to my Mac (MacBook Pro (16-inch, 2019), CPU: 2,3 GHz 8-Core Intel Core i9, GPU: Intel UHD Graphics 630 1536 MB), monitons which are fullHD (Dell P2419H)
• Another issue I saw is that if I open Firefox on my mac display (having in focus a website with scrollbar, eg wikipedia.org) and then move it to any of the external monitors, the scrollbar is not visible anymore and I can't interact with it but I can still scroll in the page. Let me know if this is not a known issue or not dependent on this bug and I'll log an issue on this as well.
• Since I can only make .mov screencasts and this one is two heavy to load it directly here, I uploaded it to Gdrive: https://drive.google.com/file/d/156DiiH9ujsg-cWrtyMpbq0L5y_DbmgFN/view?usp=sharing

And here is the about:support for my device:

Application Basics
------------------

Name: Firefox
Version: 99.0a1
Build ID: 20220225065850
Distribution ID:
Update Channel: nightly
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:99.0) Gecko/20100101 Firefox/99.0
OS: Darwin 20.6.0 Darwin Kernel Version 20.6.0: Tue Oct 12 18:33:42 PDT 2021; root:xnu-7195.141.8~1/RELEASE_X86_64
Rosetta Translated: false
Multiprocess Windows: 1/1
Fission Windows: 1/1 Enabled by default
Remote Processes: 6
Enterprise Policies: Inactive
Google Location Service Key: Found
Google Safebrowsing Key: Found
Mozilla Location Service Key: Found
Safe Mode: false

Crash Reports for the Last 3 Days
---------------------------------

Report ID: bp-363f9f94-d7a2-4552-9b88-05d1f0220225
Submitted: 2 hours ago

Nightly Features
----------------

Name: DoH Roll-Out
Version: 2.0.0
ID: doh-rollout@mozilla.org

Name: Firefox Screenshots
Version: 39.0.1
ID: screenshots@mozilla.org

Name: Form Autofill
Version: 1.0.1
ID: formautofill@mozilla.org

Name: Picture-In-Picture
Version: 1.0.0
ID: pictureinpicture@mozilla.org

Name: Web Compatibility Interventions
Version: 30.2.0
ID: webcompat@mozilla.org

Name: WebCompat Reporter
Version: 1.4.2
ID: webcompat-reporter@mozilla.org

Remote Features
---------------

Remote Processes
----------------

Type: Privileged About
Count: 1

Type: Extension
Count: 1

Type: Preallocated
Count: 3

Type: Socket
Count: 1

Add-ons
-------

Name: Add-ons Search Detection
Type: extension
Version: 2.0.0
Enabled: true
ID: addons-search-detection@mozilla.com

Name: Amazon.com
Type: extension
Version: 1.3
Enabled: true
ID: amazondotcom@search.mozilla.org

Name: Bing
Type: extension
Version: 1.3
Enabled: true
ID: bing@search.mozilla.org

Name: DuckDuckGo
Type: extension
Version: 1.1
Enabled: true
ID: ddg@search.mozilla.org

Name: Google
Type: extension
Version: 1.1
Enabled: true
ID: google@search.mozilla.org

Name: Wikipedia (en)
Type: extension
Version: 1.1
Enabled: true
ID: wikipedia@search.mozilla.org

Graphics
--------

Features
Compositing: WebRender
Asynchronous Pan/Zoom: wheel input enabled; scrollbar drag enabled; keyboard enabled; autoscroll enabled; smooth pinch-zoom enabled
WebGL 1 Driver WSI Info: CGL IsWebglOutOfProcessEnabled: 1
WebGL 1 Driver Renderer: Intel Inc. -- Intel(R) UHD Graphics 630
WebGL 1 Driver Version: 4.1 INTEL-16.5.4
WebGL 1 Driver Extensions: GL_ARB_blend_func_extended GL_ARB_draw_buffers_blend GL_ARB_draw_indirect GL_ARB_ES2_compatibility GL_ARB_explicit_attrib_location GL_ARB_gpu_shader_fp64 GL_ARB_gpu_shader5 GL_ARB_instanced_arrays GL_ARB_internalformat_query GL_ARB_occlusion_query2 GL_ARB_sample_shading GL_ARB_sampler_objects GL_ARB_separate_shader_objects GL_ARB_shader_bit_encoding GL_ARB_shader_subroutine GL_ARB_shading_language_include GL_ARB_tessellation_shader GL_ARB_texture_buffer_object_rgb32 GL_ARB_texture_cube_map_array GL_ARB_texture_gather GL_ARB_texture_query_lod GL_ARB_texture_rgb10_a2ui GL_ARB_texture_storage GL_ARB_texture_swizzle GL_ARB_timer_query GL_ARB_transform_feedback2 GL_ARB_transform_feedback3 GL_ARB_vertex_attrib_64bit GL_ARB_vertex_type_2_10_10_10_rev GL_ARB_viewport_array GL_EXT_debug_label GL_EXT_debug_marker GL_EXT_framebuffer_multisample_blit_scaled GL_EXT_texture_compression_s3tc GL_EXT_texture_filter_anisotropic GL_EXT_texture_sRGB_decode GL_APPLE_client_storage GL_APPLE_container_object_shareable GL_APPLE_flush_render GL_APPLE_object_purgeable GL_APPLE_rgb_422 GL_APPLE_row_bytes GL_APPLE_texture_range GL_ATI_texture_mirror_once GL_NV_texture_barrier
WebGL 1 Extensions: ANGLE_instanced_arrays EXT_blend_minmax EXT_color_buffer_half_float EXT_float_blend EXT_frag_depth EXT_shader_texture_lod EXT_sRGB EXT_texture_compression_rgtc EXT_texture_filter_anisotropic MOZ_debug OES_element_index_uint OES_fbo_render_mipmap OES_standard_derivatives OES_texture_float OES_texture_float_linear OES_texture_half_float OES_texture_half_float_linear OES_vertex_array_object WEBGL_color_buffer_float WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_depth_texture WEBGL_draw_buffers WEBGL_lose_context
WebGL 2 Driver WSI Info: CGL IsWebglOutOfProcessEnabled: 1
WebGL 2 Driver Renderer: Intel Inc. -- Intel(R) UHD Graphics 630
WebGL 2 Driver Version: 4.1 INTEL-16.5.4
WebGL 2 Driver Extensions: GL_ARB_blend_func_extended GL_ARB_draw_buffers_blend GL_ARB_draw_indirect GL_ARB_ES2_compatibility GL_ARB_explicit_attrib_location GL_ARB_gpu_shader_fp64 GL_ARB_gpu_shader5 GL_ARB_instanced_arrays GL_ARB_internalformat_query GL_ARB_occlusion_query2 GL_ARB_sample_shading GL_ARB_sampler_objects GL_ARB_separate_shader_objects GL_ARB_shader_bit_encoding GL_ARB_shader_subroutine GL_ARB_shading_language_include GL_ARB_tessellation_shader GL_ARB_texture_buffer_object_rgb32 GL_ARB_texture_cube_map_array GL_ARB_texture_gather GL_ARB_texture_query_lod GL_ARB_texture_rgb10_a2ui GL_ARB_texture_storage GL_ARB_texture_swizzle GL_ARB_timer_query GL_ARB_transform_feedback2 GL_ARB_transform_feedback3 GL_ARB_vertex_attrib_64bit GL_ARB_vertex_type_2_10_10_10_rev GL_ARB_viewport_array GL_EXT_debug_label GL_EXT_debug_marker GL_EXT_framebuffer_multisample_blit_scaled GL_EXT_texture_compression_s3tc GL_EXT_texture_filter_anisotropic GL_EXT_texture_sRGB_decode GL_APPLE_client_storage GL_APPLE_container_object_shareable GL_APPLE_flush_render GL_APPLE_object_purgeable GL_APPLE_rgb_422 GL_APPLE_row_bytes GL_APPLE_texture_range GL_ATI_texture_mirror_once GL_NV_texture_barrier
WebGL 2 Extensions: EXT_color_buffer_float EXT_float_blend EXT_texture_compression_rgtc EXT_texture_filter_anisotropic MOZ_debug OES_texture_float_linear WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_lose_context
Target Frame Rate: 60
GPU #1
Active: Yes
Vendor ID: 0x8086
Device ID: 0x3e9b
RAM: 0
GPU #2
Active: Yes
Vendor ID: 0x1002
Device ID: 0x7340
RAM: 0
Diagnostics
AzureCanvasBackend: skia
AzureContentBackend: skia
AzureFallbackCanvasBackend: skia
CMSOutputProfile: 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Display0: 1792x1120 scale:2.000000
Display1: 1920x1080 scale:1.000000
Display2: 1920x1080 scale:1.000000
DisplayCount: 3
Decision Log
HW_COMPOSITING:
available by default
OPENGL_COMPOSITING:
available by default
WEBRENDER:
available by default
WEBRENDER_QUALIFIED:
available by default
WEBRENDER_COMPOSITOR:
available by default
WEBRENDER_PARTIAL:
disabled by default: User disabled via pref
WEBRENDER_SHADER_CACHE:
disabled by default: Disabled by default
WEBRENDER_OPTIMIZED_SHADERS:
available by default
WEBRENDER_ANGLE:
available by default
unavailable by env: OS not supported
WEBRENDER_DCOMP_PRESENT:
available by default
disabled by user: User disabled via pref
unavailable by env: Requires Windows 10 or later
unavailable by runtime: Requires ANGLE
WEBRENDER_SOFTWARE:
available by default
WEBGPU:
disabled by default: Disabled by default




Media
-----

Audio Backend: audiounit-rust
Max Channels: 2
Preferred Sample Rate: 48000
Roundtrip latency (standard deviation): 71.12ms (0.01)
Output Devices
Name: Group
ZoomAudioDevice: zoom.us.zoomaudiodevice
MacBook Pro Speakers: builtin-internal-mic|spk
Input Devices
Name: Group
ZoomAudioDevice: zoom.us.zoomaudiodevice
MacBook Pro Microphone: builtin-internal-mic|spk
Media Capabilities
Enumerate database

Environment Variables
---------------------

MOZ_CRASHREPORTER_EVENTS_DIRECTORY: /Users/bogdan.maris/Library/Application Support/Firefox/Profiles/81xavudy.44r4/crashes/events
MOZ_CRASHREPORTER_RESTART_ARG_0: /Applications/Firefox Nightly.app/Contents/MacOS/firefox
MOZ_CRASHREPORTER_RESTART_ARG_1: -foreground
MOZ_CRASHREPORTER_DATA_DIRECTORY: /Users/bogdan.maris/Library/Application Support/Firefox/Crash Reports
MOZ_CRASHREPORTER_PING_DIRECTORY: /Users/bogdan.maris/Library/Application Support/Firefox/Pending Pings
MOZ_CRASHREPORTER_STRINGS_OVERRIDE: /Applications/Firefox Nightly.app/Contents/Resources/browser/crashreporter-override.ini
MOZ_LAUNCHED_CHILD:
MOZ_APP_SILENT_START:
XRE_PROFILE_PATH:
XRE_PROFILE_LOCAL_PATH:
XRE_RESTARTED_BY_PROFILE_MANAGER:
MOZ_CRASHREPORTER_RESTART_ARG_2:
XRE_START_OFFLINE:
XRE_BINARY_PATH:

Experimental Features
---------------------

about:home startup cache (browser.startup.homepage.abouthome_cache.enabled): true
Cookies: SameSite=Lax by default (network.cookie.sameSite.laxByDefault): true
Cookies: SameSite=None requires secure attribute (network.cookie.sameSite.noneRequiresSecure): true
Cookies: Schemeful SameSite (network.cookie.sameSite.schemeful): true
CSS: Constructable Stylesheets (layout.css.constructable-stylesheets.enabled): false
CSS: Masonry Layout (layout.css.grid-template-masonry-value.enabled): true
Developer Tools: Compatibility Panel (devtools.inspector.compatibility.enabled): true
Developer Tools: Service Worker debugging (devtools.debugger.features.windowless-service-workers): false
Firefox 100 User-Agent String (general.useragent.forceVersion100): false
Media: JPEG XL (image.jxl.enabled): false
Address Bar: show results during IME composition (browser.urlbar.keepPanelOpenDuringImeComposition): false
Web API: WebGPU (dom.webgpu.enabled): false
WebRTC Global Mute Toggles (privacy.webrtc.globalMuteToggles): false

Remote Experiments
------------------

Firefox 100 User-Agent Nightly 97-99: version-default

Important Modified Preferences
------------------------------

browser.contentblocking.category: standard
browser.search.region: RO
browser.sessionstore.upgradeBackup.latestBuildID: 20220225065850
browser.startup.homepage_override.buildID: 20220225065850
browser.startup.homepage_override.mstone: 99.0a1
browser.urlbar.placeholderName: Google
browser.urlbar.quicksuggest.migrationVersion: 2
browser.urlbar.quicksuggest.scenario: history
doh-rollout.balrog-migration-done: true
doh-rollout.doneFirstRun: true
doh-rollout.home-region: RO
extensions.lastAppVersion: 99.0a1
general.useragent.handledVersionExperimentEnrollment: true
idle.lastDailyNotification: 1643973354
media.gmp-gmpopenh264.abi: x86_64-gcc3
media.gmp-gmpopenh264.lastUpdate: 1643964084
media.gmp-gmpopenh264.version: 1.8.1.1
media.gmp-manager.buildID: 20220203190329
media.gmp-manager.lastCheck: 1643964084
media.gmp-widevinecdm.abi: x86_64-gcc3
media.gmp-widevinecdm.lastUpdate: 1643964085
media.gmp-widevinecdm.version: 4.10.2391.0
media.gmp.storage.version.observed: 1
places.database.lastMaintenance: 1643973354
privacy.purge_trackers.date_in_cookie_database: 0
privacy.purge_trackers.last_purge: 1643973354768
privacy.sanitize.pending: [{"id":"newtab-container","itemsToClear":[],"options":{}}]
security.remote_settings.crlite_filters.checked: 1643966437
security.remote_settings.intermediates.checked: 1643964084
security.sandbox.content.tempDirSuffix: 02b78722-6b22-4bbf-ae88-4a930eb2b821
services.sync.declinedEngines:
storage.vacuum.last.index: 0
storage.vacuum.last.places.sqlite: 1643973354

Important Locked Preferences
----------------------------

fission.autostart.session: true

Places Database
---------------

Accessibility
-------------

Activated: false
Prevent Accessibility: 0

Library Versions
----------------

NSPR
Expected minimum version: 4.33
Version in use: 4.33

NSS
Expected minimum version: 3.76 Beta
Version in use: 3.76 Beta

NSSSMIME
Expected minimum version: 3.76 Beta
Version in use: 3.76 Beta

NSSSSL
Expected minimum version: 3.76 Beta
Version in use: 3.76 Beta

NSSUTIL
Expected minimum version: 3.76 Beta
Version in use: 3.76 Beta

Sandbox
-------

Content Process Sandbox Level: 3
Effective Content Process Sandbox Level: 3
Win32k Lockdown State for Content Process: Win32k Lockdown disabled -- Operating system not supported

Startup Cache
-------------

Disk Cache Path: /Users/bogdan.maris/Library/Caches/Firefox/Profiles/81xavudy.44r4/startupCache/startupCache.8.little
Ignore Disk Cache: false
Found Disk Cache on Init: true
Wrote to Disk Cache: false

Internationalization & Localization
-----------------------------------

Application Settings
Requested Locales: ["en-US"]
Available Locales: ["en-US"]
App Locales: ["en-US"]
Regional Preferences: ["en-GB","en-RO","ro-RO"]
Default Locale: "en-US"
Operating System
System Locales: ["en-GB","en-RO","ro-RO"]
Regional Preferences: ["en-GB","en-RO","ro-RO"]

Remote Debugging (Chromium Protocol)
------------------------------------

Accepting Connections: false
URL:

Printing
--------

Modified print settings
-----------------------

Another issue I saw is that if I open Firefox on my mac display (having in focus a website with scrollbar, eg wikipedia.org) and then move it to any of the external monitors, the scrollbar is not visible anymore and I can't interact with it but I can still scroll in the page. Let me know if this is not a known issue or not dependent on this bug and I'll log an issue on this as well.

This would be totally unrelated. Can you file a separate bug for this?

Thank you.

In which monitor are you opening Firefox? Does the behavior wrt this bug change depending on that?

Set release status flags based on info from the regressing bug 1741830

Ah, I could repro, fix incoming.

I missed this one in https://phabricator.services.mozilla.com/D138039 :(

The old calculation based on device pixels might be correct, but it sure was
sketchy. This is more straight-forward / easier to follow.

Depends on D139777

https://hg.mozilla.org/mozilla-central/rev/8c1888a6aef9
https://hg.mozilla.org/mozilla-central/rev/2b61ceef478a

I verified that using my setup this is no longer an issue on the Latest Nightly.