[bobsingor]

A signature field's widget had no drawing until a signature was drawn into it. EPDFAnnot_GenerateFormFieldAP returned false for /FT /Sig and for a widget in no field, and attaching a widget to a signature field baked nothing. The field's /MK colours and /BS were written but never drawn, so a viewer offered a background and a border that never showed. Acrobat's signature field properties have the same four settings (border colour, fill colour, line thickness, line style), and Acrobat draws the empty field with them.

Now:

• CPDF_AppearanceGenerator::GenerateBoxFormAP draws a widget that shows only its box: its background (/MK /BG) and border (/MK /BC, /BS), as a text field draws them, as a new normal appearance stream. It makes a new stream, as a check box's generator does, because the old /N may be another widget's too, or a drawing someone placed in this one.
• The text field's box moved into WriteFieldBox, which both use. The text field's output is unchanged.
• EPDFAnnot_GenerateFormFieldAP draws the box for a /Sig widget and for a widget in no field. It replaces whatever the widget shows, a signature's mark included. Keeping a mark is the caller's decision: the engine never calls it on a widget with a mark in it.
• EPDFForm_AttachWidget bakes the box for a signature field (family 7), as it bakes every other family.
• EPDFAnnot_GenerateFormFieldAP refuses an annotation that isn't a widget. A non-widget has no /FT, so with the no-field branch it would otherwise get a box.

RegenerateFieldAppearances and the repair bake still skip signature fields.

The engine side is embedpdf/embed-pdf-viewer#982: it decides when to draw, and locks a field's style once a mark is in it. Pin this there after release.

Tests

• New embedder tests, recorded as layer-mode writers:

  • AttachToSignatureFieldDrawsItsBox (7 objects): the box in the field's /MK colours.
  • SignatureFieldBoxRedrawsInItsNewStyle (8): an empty box, then the new background after a restyle.
  • WidgetInNoFieldDrawsItsBox (3).
  • GenerateFormFieldAPRefusesAnotherSubtype (2): a square gets no appearance.

  In layer mode the four pass, with no unknown writers.

• On macOS:

  • Embedder suite: 1,773 pass and 12 fail. The same 12 fail without the change, built from eeac787c0: progressive render, text render, RenderHelloWorldWithFlags and CaptionAppearanceGolden. RenderFormsWithPause is off by the same 19 pixels in both.
  • Unit tests: all 1,320 pass.
  • Pixel tests: 51 fail and 2 surprise, and the base fails the same ones by the same pixels.

• Linux CI hasn't run yet.