fix: write a DataGrid 2 column's Visible expression

[ako]

Problem

A DataGrid 2 column's Visible: expression was silently dropped. These spellings all passed check, and exec reported "Created page", but the column was stored with Expression true, i.e. always visible:

• Visible: $showPrices
• Visible: if … then … else …
• visible: not(…)
• Visible: [cond]

Only the old quoted spelling Visible: '<expr>' was kept. On the ALTER side:

• alter page … set (Visible: <expression>) on grid column(…) was refused with "column property VisibleIf not found".
• An inserted or replaced column also dropped Visible: false.

Cause

The visitor stores every expression spelling of Visible under the key VisibleIf, the key a page widget's conditional visibility reads. Only plain values stay under Visible. The column writers read only visible/Visible:

• Widget engine (create): looks up the schema key plus types.ItemPropertyAliases, which had no VisibleIf alias.
• ALTER set: resolveColumnPropertyKey uses the same table, so it refused the key.
• ALTER insert/replace: buildColumnSpecFromAST passes the properties to widgetobj.BuildDataGrid2Column, which reads the exact key "Visible" and only as a string, so a bool false was dropped too.

This is the mirror image of bug-tests/widget-visible-expression.mdl, where page widgets read VisibleIf and dropped Visible.

Fix

• Alias: types.ItemPropertyAliases gets visible ← VisibleIf. This fixes create and ALTER set. WidgetDefGeneratorVersion goes to 18, so existing projects regenerate their datagrid def.
• Insert/replace: columnSpecProperties gives the column builder Visible as the string it reads (from VisibleIf, or a static false). It also passes DynamicCellClass through in any letter case.
• Describe: prints the bare expression (via widgetConditionMDL, as page widgets do), and the output round-trips.
• MDL-WIDGET43 (new error): a column's visible expression is evaluated once for the grid. Its widget schema declares no dataSource, unlike columnClass, so there is no row object. On Mendix 11.14.0, mx check reports CE0117 for every column whose Visible uses $currentObject; those columns only appeared to work because the value was dropped. check now refuses this on create, insert and set, and suggests a page variable or parameter.

Verification

• Tests first, each seen failing with the symptom before the fix:
  • columnSpecProperties for each spelling (the quoted form and true are controls)
  • the def generator's visible alias
  • resolveColumnPropertyKey("VisibleIf")
  • MDL-WIDGET43 on create, insert and set (a textbox's $currentObject Visible is the control)
  • the describe round trip
• End to end on copies of ako/TestApp (Mendix 11.14.0):
  • Before the fix, the page data stored true for each expression spelling.
  • After it, every spelling is stored on create, ALTER set and ALTER insert, using a page variable.
  • mx check reports 0 errors.
  • The describe output re-execs to an identical page.
  • exec refuses a $currentObject column Visible as MDL-WIDGET43.
• make build && make test && make lint && make check-mdl and make check-findings all pass.

Also

• A regression script, mdl-examples/bug-tests/datagrid-column-visible-expression.mdl.
• The page.column syntax help and the quick reference, which showed the quoted spelling.
• The 03-page-examples.mdl column example, which now uses visible: $showStockColumn.
• A finding, a paragraph on docs-wiki/bug-patterns/silent-property-drop.md, and a CHANGELOG entry.

Not covered: Visible: Attr in (…) on a column is still ignored without a warning. That form lets the attribute decide visibility, and a column has no row to read it from.

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