Skia: broken multi-line text input

[jpnurmi]

Originally posted by @jpnurmi in https://github.com/slint-ui/slint/issues/3588#issuecomment-1742847367

Multiline editing is somehow broken with Skia. Sometimes (at the end of any non-empty line?) you need to press enter twice to insert a new line. Once you type in the next character, the extra line break appears.

"Center" + "↵"	+ "↵"	+ "n"

Latest master (e44f70c9dfcf0379390b5f8886764e38c2bf9c25), macOS, winit-skia

[jpnurmi]

Similarly, while erasing, an extra backspace press is required.

https://github.com/slint-ui/slint/assets/140617/6367572d-4bdc-43b5-9f07-ca972784dcab

[jpnurmi]

The text boxes returned by SkParagraph::getRectsForRange() that are used for calculating the cursor rectangle don't quite match with SkParagrap::getLineMetrics(), at least when there's a singular trailing newline. For example, "Hello" and "Hello\n" are both presented as a single identical text box.

"Hello"

• boxes (1):
  • l=0.000000, t=-0.300000, r=35.490002, b=16.000000
• lines (1):
  • 0: "Hello" (0..5): l=0.000000, t=-0.296875, r=35.490002, b=16.000000

    "Hello\n"

• boxes (1):
  • l=0.000000, t=-0.300000, r=35.490002, b=16.000000
• lines (2):
  • 0: "Hello\n" (0..6): l=0.000000, t=-0.296875, r=35.490002, b=16.000000
  • 1: "\n" (5..6): l=0.000000, t=15.703125, r=0.000000, b=32.000000

    "Hello\n\n"

• boxes (3):
  • l=0.000000, t=-0.300000, r=35.490002, b=16.000000
  • l=0.000000, t=15.700000, r=0.000000, b=32.000000
  • l=0.000000, t=15.700000, r=35.490002, b=32.000000
• lines (3):
  • 0: "Hello" (0..5): l=0.000000, t=-0.296875, r=35.490002, b=16.000000
  • 1: "\n" (6..7): l=0.000000, t=15.703125, r=0.000000, b=32.000000
  • 2: "\n" (6..7): l=0.000000, t=31.703125, r=0.000000, b=48.000000

    "Hello\nworld\n"

• boxes (3):
  • l=0.000000, t=-0.300000, r=35.490002, b=16.000000
  • l=35.490002, t=-0.300000, r=38.549999, b=16.000000
  • l=0.000000, t=15.700000, r=38.549999, b=32.000000
• lines (3):
  • 0: "Hello" (0..5): l=0.000000, t=-0.296875, r=35.490002, b=16.000000
  • 1: "world\n" (6..12): l=0.000000, t=15.703125, r=38.549999, b=32.000000
  • 2: "\n" (11..12): l=0.000000, t=31.703125, r=0.000000, b=48.000000

    "Hello\n \n"

• boxes (3):
  • l=0.000000, t=-0.300000, r=35.490002, b=16.000000
  • l=0.000000, t=15.700000, r=4.450000, b=32.000000
  • l=4.450000, t=15.700000, r=35.490002, b=32.000000
• lines (3):
  • 0: "Hello" (0..5): l=0.000000, t=-0.296875, r=35.490002, b=16.000000
  • 1: " \n" (6..8): l=0.000000, t=15.703125, r=0.000000, b=32.000000
  • 2: "\n" (7..8): l=0.000000, t=31.703125, r=0.000000, b=48.000000

https://fiddle.skia.org/c/8ed057a649687bf916581fe0b8ebfad0