fix(pages): write a null TitleOverride, unblanking every popup caption (#812)

[ako]

Fixes mendixlabs#812.

The bug

Every Forms$FormSettings / Forms$PageSettings mxcli wrote carried an empty Microflows$TextTemplate as its TitleOverride, where Studio Pro writes null.

An empty template is not the absence of an override — it is an override, to the empty string. So every popup opened by an mxcli-authored button or Show Page action rendered with a blank caption and just the ×. The reporter found 10 broken popups (all mxcli-authored) against 58 correct ones from Studio Pro / marketplace modules in the same project.

Reproduced and verified in a browser

A scratch Mendix 11.12.2 project, a popup page titled Order Details Popup, and an mxcli-authored button that opens it. Same app, same page, booted via mxcli run --local; the only variable is which build wrote the page. Driven with Playwright, reading the caption out of the modal header:

Build	TitleOverride in BSON	headerText	caption
this PR	null	"×\nOrder Details Popup"	"Order Details Popup"
pre-fix	empty Microflows$TextTemplate	"×"	"" — blank

That is the reported symptom reproduced exactly, and closed.

The build-level claim behind the original workaround is also disproved. The old comments said Studio Pro rejects a null embedded object on load. With the null in place, on 11.12.2:

mxcli docker check -p app.mpr   →  "The app contains: 0 errors."
mxcli docker build -p app.mpr   →  "Build complete."

A second defect the first one hid

The same unconditional write discarded an override the author did ask for. show page M.P with title = 'X' parses, the visitor sets ast.ShowPageStmt.Title, the builder sets ShowPageAction.OverridePageTitle — and no writer ever read that field. Before this change, grep -rn OverridePageTitle matched exactly two lines: the struct field and its assignment.

Both cases produced identical BSON, which is precisely why nobody noticed the second half. Both now round-trip.

Two traps, both now in the symptom table

1. A comment citing the wrong issue. The must be non-nil comments justified themselves with issue mendixlabs#295, which was about Forms$PageVariable on a PageParameterMapping — a different field. The conclusion was generalised to TitleOverride without being tested, and the runs above show it is false. This repo's own .claude/skills/debug-bson.md already documented {Key: "TitleOverride", Value: nil} as the correct shape.

Two existing tests asserted the incorrect behaviour. They are inverted here, with the reasoning recorded inline so the reversal is not silently re-reverted by someone reading the old comment.

2. codec.RegisterTypeDefaults overwrites rather than merges. Forms$FormSettings was registered twice — in microflow_write.go and widget_write.go. Registrations resolve by init order, so the NullFields entry I added beside the microflow writer was clobbered and the null never reached the output. Caught only by dumping the BSON rather than trusting the passing test. Consolidated to a single registration, with a comment at the surviving site.

Scope

The microflow Show Page and widget-action paths, where the evidence is. Navigation writes the same empty template at three sites and is deliberately left alone — the reported evidence is about popups, and changing navigation on an analogy is exactly the mistake that caused this bug in the first place.

Unit-level coverage

show page P;                        → TitleOverride: null
show page P with title = 'X';       → Microflows$TextTemplate containing 'X'
actionbutton (action: show_page P)  → TitleOverride: null

Reverting the fix makes the new tests fail with exactly the BSON shape quoted in the issue. Repro script added at mdl-examples/bug-tests/812-showpage-title-override.mdl. Full Go suite green.

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https://claude.ai/code/session_012XR649rKk68z6gBpngu6MA