Process container: permissive UI policy values are not grantable

[theelliotm]

Relevant area(s)

Windows

Brief description of your issue

Permissive UI policy values are not grantable in the process container. Setting processContainer.ui.desktopSystemControl to true, processContainer.ui.systemSettings to display, parameters, or all, or ui.clipboard to a permissive value still leaves the capability blocked inside the sandbox.

This is 36 failing assertions in the 2026-09-23 nightly run, on every Windows job:

• prerelease process-t1 (6, clipboard behaves differently there, see Process container: clipboard restrictions are not enforced on the prerelease Windows build #1246)
• 25h2 process-t1 (10)
• 24h2 process-t1 (10)
• 23h2 process-t3 (10)

Two things narrow this down:

• The restrictive direction works. Only granting fails.
• processContainer.ui.isolation is fully bidirectional on all four hosts. isolation=desktop -> HANDLES=allowed passes, isolation=handles -> HANDLES=blocked passes, and the negative control passes. So the UI plumbing works in general.

resolve_ui_restrictions (src/core/wxc_common/src/ui_policy.rs:68-122) computes the mask correctly, so the loss is below that layer. It reproduces on both the job object path (T3) and the PSEC spec path (T1), which suggests it is not specific to either mechanism.

Steps to reproduce

1. Run tests/scripts/run_processcontainer_ui_policy_matrix_test.ps1 on any supported Windows host.
2. Watch phase P4e.

Expected behavior

docs/process-container/UIPolicy_Schema.md:95 and docs/process-container/os-version-support.md:177-182 document all three of these as supported on every build, so a permissive value should leave the capability usable.

Actual behavior

The UI probe reports the capability blocked no matter what was configured:

P4e :: pcUi.desktopSystemControl=true -> DESKTOP=allowed (expected=allowed; got=blocked; full=DESKTOP=blocked)
P4e :: pcUi.systemSettings=all -> SYSTEMPARAMETERS=allowed (expected=allowed; got=blocked)
P4e :: pcUi.systemSettings=display -> DISPLAYSETTINGS=allowed (expected=allowed; got=blocked)
P4e :: ui.clipboard=all -> READCLIPBOARD=allowed (expected=allowed; got=blocked)
P4e :: baseline: a fully permissive UI policy grants every UI capability
       (still denied: READCLIPBOARD, WRITECLIPBOARD, SYSTEMPARAMETERS, DISPLAYSETTINGS, DESKTOP)

Either enforcement is wrong or the support matrix is. If the matrix is wrong, the docs should say these are always denied and the tests should be updated to match.