fix(pages): flow-argument mappings crash Studio Pro's Changes panel and bind the wrong slot (mendixlabs/mxcli#1317)

[ako]

Fixes mendixlabs#1317.

Reported symptom

After alter page LearningJourney.LearningJourney_Details { set ('onClickAction': call microflow LearningJourney.ACT_ExcludeItemStep(LearningJourney = $LearningJourney)) on pDSLink_ButtonAndTracking8 }; on Studio Pro 11.12.2, opening the change in the Changes panel throws:

System.InvalidOperationException: Objects with ID [redacted] of type Forms$MicroflowParameterMapping do not have the same properties. baseNames = Expression, Parameter, Variable; newNames = Parameter, Expression

Three defects (five commits)

1. Variable key omitted on an Expression-bound argument (the crash). Studio Pro writes Expression, Parameter and Variable on every Forms$MicroflowParameterMapping / Forms$NanoflowParameterMapping, with the unused slot set to null. mxcli left Variable unset, and the encoder drops an unset part. Because the stored $ID is carried onto the rewritten mapping, the merge library sees one element in two shapes. Same class as Studio Pro crashes when opening Changes details after ALTER ENTITY ... MODIFY ATTRIBUTE mendixlabs/mxcli#1180.
   Fix: register Variable as a NullFields default on both types (mdl/backend/modelsdk/widget_write.go). This covers every writer: actions, nanoflow actions and nanoflow data sources.
2. alter page … set built the action with an empty parameter scope. So $LearningJourney, a page parameter, was stored as the Expression "$LearningJourney" instead of Variable → Forms$PageVariable. That is the Page parameter not wired in nanoflow argument when button is inside a dataview bound to a different parameter CE1571 mendixlabs/mxcli#1140 form, which Studio Pro does not bind (CE1571).
   Fix: convertASTAction seeds paramScope, page variables and isSnippet from the mutator, as insert/replace already did.
3. alter snippet … insert/replace bound a snippet-parameter argument as a page parameter. buildWidgetsFromAST / buildColumnSpecsFromAST never set isSnippet.
   Fix: set it from mutator.ContainerType().

Evidence

Tests written first:

• widget_flow_param_null_variable_test.go: failed with mapping keys = Expression, Parameter.
• TestAlterPage_SetNamedAction_PageParamBindsThroughVariable: failed with kind "".
• TestAlterSnippet_InsertedFlowArgBindsSnippetParameter: failed with kind "parameter". Its page-container control passes both ways.

Reverting fix 1 made its test fail again with the reported key set.

Real tool, mxbuild 11.12.2, buggy and fixed builds side by side:

Defect	Check	Buggy build	Fixed build
1	mx diff base.mpr new.mpr out.mpr (same comparison code as the Changes panel)	exit 129, Objects with ID … of type Forms$MicroflowParameterMapping do not have the same properties. baseNames = Parameter, Expression, Variable; newNames = Parameter, Expression	exit 0
2	bson dump of the mapping	Expression: "$LearningJourney", no Variable	Variable: Forms$PageVariable{PageParameter: "LearningJourney"}; the literal argument carries Variable: null
3, inserted widget	mx check	1 error: [CE0115] … at Action button 'btnInsert'	0 errors; created, inserted and set buttons all bind SnippetParameter
3, inserted grid column	mx check	1 error: [CE0115] … at Action button 'btnCol', binding PageParameter: "Order"	0 errors, binding SnippetParameter: "Order"

mx check reports 0 errors for defects 1 and 2 on both builds, so a build does not catch either.

Not verified

• I didn't open Studio Pro's Changes panel itself; mx diff stands in for it. My baseline was written by the fixed build, which writes the same three-key shape as Studio Pro.

Regression coverage

• mdl-examples/bug-tests/1317-flow-arg-mapping-null-variable.mdl
• mdl-examples/bug-tests/1317-alter-snippet-insert-flow-arg.mdl: the inserted widget and the inserted grid column. The whole file runs on a fresh 11.12.2 project at 0 errors.
• Findings appended to mdl-backend.jsonl and mdl-executor.jsonl. The main point: mx diff reproduces Changes-panel crashes without Studio Pro.
• make build, make test, make lint, make check-mdl and make check-findings all pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01HamahFUo9MM2xuaNmm5mhr