MSW: Defer dark mode system colour change event (PR #26698)

[Steve Cornett]

These changes address an issue with unreliable WM_SETTINGCHANGE "ImmersiveColorSet" messages upon switching between dark and light mode. See #26692.

Normally when the system switches between modes, Windows sends two WM_SETTINGCHANGE messages - one before the switch and one after. The problem addressed is that the second message never appears. The reason is unknown. It may be due to a timeout, since this message is broadcast using SendMessageTimeout().

The fix has two parts.

1. When the WM_SETTINGCHANGE is received, instead of immediately issuing a wxSysColourChangedEvent, we instead post WM_SYSCOLORCHANGE to defer that action until after the mode switch is finished. This guarantees we issue at least one wxSysColourChangedEvent after the mode switch.

2. The change above exposed a latent bug in the dark mode initialization. The variable gs_wasActiveOnStartup was not initialized correctly because ShouldAppsUseDarkMode() did not return an accurate value until a window was created. This problem was masked by having two WM_SETTINGCHANGE messages, one with IsActive() true and one false. This caused the comparison IsActive() != gs_wasActiveOnStartup to be true one time despite gs_wasActiveOnStartup begin wrong. The solution is to call the undocumented API RefreshImmersiveColorPolicyState() so that ShouldAppsUseDarkMode() returns the correct value.

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You can view, comment on, or merge this pull request online at:

  https://github.com/wxWidgets/wxWidgets/pull/26698

Commit Summary

• ccd0df3 Defer dark mode system colour change event

File Changes

(2 files)

• M src/msw/darkmode.cpp (15)
• M src/msw/window.cpp (11)

Patch Links:

• https://github.com/wxWidgets/wxWidgets/pull/26698.patch
• https://github.com/wxWidgets/wxWidgets/pull/26698.diff

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