Batch cherry-pick: 9 h3xds1nz upstream PRs (clean apply onto if/main)

[oysteinkrog]

Summary

Batch cherry-pick of 9 cleanly-applied h3xds1nz PRs from dotnet/wpf upstream that haven't been picked up there. Picks up where Phase B left off — these are the perf/cleanup PRs h3xds1nz opened after his last upstream-merged work in May 2025.

Stacked on: #1 (crossgen2 R2R) and #2 (CommandManager typeof fix + denylist) — both merged. Branch is rebased onto current if/main.

Cherry-picked PRs (all clean apply)

Upstream PR	Title (commit subject)
dotnet#9977	Define new constructs out of generated file
dotnet#9982	Replace ArrayList in HwndHost with generic List
dotnet#9983	Remember to not make FrugalStructList readonly, dummy
dotnet#10003	Fix invalid value write to CellsPanelHorizontalOffset and infinite loop
dotnet#10248	Adjust style to be on par with other change
dotnet#10310	Remove SecurityHelper from PBT/RF compilation
dotnet#10381	Fix InvalidCastException in Can/ConvertTo
dotnet#10664	Remove empty NativeMethods
dotnet#10711	Replace Box(true) -> TrueBox

All applied with git cherry-pick -x … so the upstream SHA is recorded in each commit.

Dropped from initial batch

• [StyleCleanUp] Avoid legacy suppression format, use GlobalSuppressions (IDE0077) dotnet/wpf#10668 — "Remove from .editorconfig" (drops IDE0077.severity = suggestion). Upstream's PR is safe because it's stacked on a separate GlobalSuppressions.cs modernization (commit 980642f4d) we haven't picked. Dropping the override without that modernization makes IDE0077 fire as error against existing legacy-format SuppressMessageAttribute entries. Re-pick once we adopt the modernization commit.

Deferred (50 PRs with conflicts)

The remaining 50 of the 60 candidate PRs touch files we've diverged on. They need per-PR judgment.

Modify/delete on already-removed files (5 PRs)

• 9990 (GenericsInstances.cs), 10009 (PartialArray.cs), 10245 (MessageWindow.cs) — upstream removes; our fork kept
• 10543 (WeakReferenceCache.cs) — already deleted in our fork; drop the upstream PR
• 10635 (TreeBuilder.cs) — upstream removes; our fork kept

.editorconfig overlaps (2 PRs)

• 10613, 10647

Content conflicts (43 PRs)

Files where our fork has accumulated edits that conflict with upstream's edits. Full list captured in cherrypick-results.txt (local).

Test plan

• CI: build green on this branch (after dropping [StyleCleanUp] Avoid legacy suppression format, use GlobalSuppressions (IDE0077) dotnet/wpf#10668)
• Bins still R2R'd (crossgen2 step from ci: R2R-compile staged DLLs (crossgen2) before nupkg pack #1)
• Smoke: consume the resulting nupkg in MotionCatalyst, verify no startup regression
• Manual review of each commit's (cherry picked from commit ...) trailer

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