A non-web protocol in a formatter href silently drops the whole element

Symptom

A gallery card (or a formatted cell) contains an "Open in app" link built with the Office URI scheme:

{
  "elmType": "a",
  "attributes": {
    "href": "=if([$File_x0020_Type] == 'docx', 'ms-word:ofe|u|', if([$File_x0020_Type] == 'xlsx', 'ms-excel:ofe|u|', '')) + 'https://contoso.sharepoint.com' + [$FileRef]"
  }
}

After deploying, items whose expression produces the ms-word: / ms-excel: / ms-powerpoint: prefix render as blank cards — not a broken link, not a missing icon, the entire element tree is gone. Items where the expression yields a plain https:// URL (here: PDFs, which got the empty prefix) render normally. There is no console error and the formatter itself saves and reads back fine.

That per-item split is the diagnostic tell: the same JSON, some rows fine, some rows empty, and the difference correlates exactly with what the href expression evaluates to.

Cause

Attribute values in list formatting are sanitized at render time. href accepts ordinary web URLs; an application protocol fails that validation and SharePoint's reaction is to drop the element — including every child — rather than render the anchor without the attribute. Because the href is an expression, the drop is evaluated per item, which is why only some rows disappear.

Fix

Don't fight the sanitizer. For "open in desktop app", delegate to the native context menu, which has Open in app built in:

{
  "elmType": "button",
  "attributes": { "title": "More actions" },
  "customRowAction": { "action": "openContextMenu" },
  "children": [ { "elmType": "span", "attributes": { "iconName": "MoreVertical" } } ]
}

Keep real links strictly on web URLs — open in browser and download are safe:

{ "elmType": "a", "attributes": { "href": "[$FileRef]", "target": "_blank" } }
{ "elmType": "a", "attributes": { "href": "=[$FileRef] + '?download=1'" } }

Notes

• Deploy risky attribute changes on one element first. Because the failure mode is silent removal, a bad href shipped to every card looks like a completely broken view and sends you hunting in the wrong places (schema, tileProps, CSS) before you suspect a link.
• The same wholesale-drop behaviour is worth assuming for any attribute the sanitizer dislikes — treat "element missing, no error" as an attribute-validation symptom.
• Related: Gallery cards render from tileProps — the other way a card goes blank with no error.