Light Switch: add Follow Screen Brightness mode - #49625
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Adds a new FollowBrightness schedule mode to LightSwitch that monitors display brightness (via WMI WmiMonitorBrightness) and automatically switches the system theme based on a user-defined threshold. Changes: - BrightnessObserver.h: header-only polling observer that queries the WMI WmiMonitorBrightness class every 5 seconds and fires a callback when brightness changes. Works for laptop/tablet integrated displays including ALS-driven adjustments. - ScheduleMode enum: adds FollowBrightness value with ToString/FromString - LightSwitchConfig: adds brightnessThreshold field (0-100, default 90) - SettingId enum: adds BrightnessThreshold entry - LightSwitchSettings: loads brightnessThreshold from settings.json - LightSwitchStateManager: adds OnBrightnessChange(int) handler; EvaluateAndApplyIfNeeded applies light mode when brightness >= threshold - LightSwitchService: starts/stops BrightnessObserver when mode changes; adds wbemuuid.lib to linker dependencies - Settings UI: adds Follow Screen Brightness mode to the combo box with a slider control for the threshold (1-100%, default 90%) - Resources.resw: adds English strings for the new mode and slider Closes microsoft#49624 Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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- Settings UI: Added the missing FollowBrightnessState to VisualStateGroups in LightSwitchPage.xaml so the slider actually appears when selecting the Follow Screen Brightness mode. - BrightnessObserver.h: Optimized the WMI polling logic. Instead of tearing down and completely rebuilding the COM/WMI connection every 5 seconds (which causes high CPU/handle churn in WmiPrvSE.exe), the connection is initialized once and kept open across the polling loop. - LightSwitchStateManager.cpp: Fixed a bug where any minor brightness fluctuation would clear the manual override state. Now it only clears the override if the brightness change actually crosses the configured threshold.
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@microsoft-github-policy-service agree |
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Hi @dkocich, Some recommended reading: #28769 and https://github.com/microsoft/PowerToys/blob/main/CONTRIBUTING.md. Please add screenshots and/or a screen recording demonstrating the updated behavior. This repository also have a template for PR descriptions, please update yours to match it: https://github.com/microsoft/PowerToys/blob/main/.github/pull_request_template.md |
dkocich
marked this pull request as draft
August 2, 2026 21:29
Fixes follow-brightness UX and persistence issues in local testing: - apply schedule-mode visual state on page load and after settings file reload so the brightness threshold slider is shown consistently - include BrightnessThreshold in LightSwitchSettings.Clone() so value survives view-model/repository reload paths - avoid Windows min/max macro clash in service settings parsing by replacing std::min/std::max clamp with explicit bounds checks
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hi @jiripolasek, thanks for the info. The description and code are updated. Here is a small GIF showing how it works. Functionality looks stable during my testing in the debug build. I tested both on a laptop (HP ZBook G1A) alone and on a secondary monitor screen only, where I trigger keyboard shortcuts to lower/raise brightness levels 
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dkocich
marked this pull request as ready for review
August 4, 2026 16:26
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Summary of the Pull Request
Implements a new Follow Screen Brightness schedule mode for LightSwitch, as requested in #49624.
When selected, LightSwitch polls the display brightness (via WMI
WmiMonitorBrightness) and automatically switches the Windows theme based on a user-configurable threshold:This is especially useful for outdoor work where an ambient light sensor (ALS) automatically pushes the display to maximum brightness to compensate for glare — a time-based schedule can't handle this, but brightness-based switching does it automatically.
PR Checklist
Detailed Description of the Pull Request / Additional comments
C++ backend
BrightnessObserver.h— new header-only observer that pollsWmiMonitorBrightnessevery 5 seconds and fires a callback on brightness changes. Works for laptop/tablet integrated displays driven by ALS or manual adjustments. It initializes COM and the WMI connection once and keeps it open to prevent CPU/handle churn inWmiPrvSE.exe.ScheduleMode— newFollowBrightnessenum value withToString/FromStringround-tripLightSwitchConfig— newbrightnessThresholdfield (0–100, default 90)SettingId— newBrightnessThresholdentryLightSwitchSettings.cpp— loadsbrightnessThresholdfromsettings.jsonLightSwitchStateManager— newOnBrightnessChange(int)handler;EvaluateAndApplyIfNeededapplies light/dark based on threshold; manual override is cleared only when the brightness crosses the threshold.LightSwitchService.cpp— starts/stopsBrightnessObserverwhen mode is switched to/fromFollowBrightness; linkswbemuuid.libSettings UI (C#/XAML)
BrightnessThresholdproperty inLightSwitchPropertiesandLightSwitchViewModelFollowBrightnessState)Validation Steps Performed
settings.jsonSettings.UI.UnitTests/ViewModelTests/LightSwitch.cs(for FollowBrightness clone persistence, BrightnessThreshold clamping, Setting schedule mode to FollowBrightness)