Default cursor thickness is too thin on Retina displays

[injust]

Issue Description

On a Retina display, the bar or block_hollow cursors seem too thin at the default cursor thickness (0.5pt = 1px).

This is a good workaround:

adjust-cursor-thickness = 1

However, on a non-Retina external display, the cursor already looks fine without adjust-cursor-thickness, and the above config makes the cursor too thick.

Originally posted by @injust in #2363 (comment)

Expected Behavior

Default to a good-looking cursor thickness on both Retina and non-Retina displays.

1pt on both (so 2px on Retina and 1px on non-Retina) seems to look the best on my 2019 13" Intel MBP.

Actual Behavior

On my MBP built-in Retina display, depending on where the cursor is relative to the screen, some edges of the block appear to be grey or dim white. This happens at any font size.

I use the More Space (1680x1050) resolution instead of Default (1440x900), which seems to cause the issue to occur more frequently.

Reproduction Steps

1. Open Ghostty
2. Spam open a lot of small splits

Ghostty Logs

No response

Ghostty Version

Ghostty 1.1.3

Version
  - version: 1.1.3
  - channel: stable
Build Config
  - Zig version: 0.13.0
  - build mode : builtin.OptimizeMode.ReleaseFast
  - app runtime: apprt.Runtime.none
  - font engine: font.main.Backend.coretext
  - renderer   : renderer.Metal
  - libxev     : main.Backend.kqueue

OS Version Information

macOS Sequoia 15.6.1

Minimal Ghostty Configuration

cursor-style = block_hollow
shell-integration-features = no-cursor

Additional Relevant Configuration

No response

I acknowledge that:

• I have reviewed the FAQ and confirm that my issue is NOT among them.
• I have searched the Ghostty repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.

[ChrHorn]

This seems to be a general issue with fractional scaling. Ran into the same issue on Linux, Wayland.

Here is a screenshot (left: Ghostty, right: Alacritty) with 1.15 scaling factor:

Ghostty renders the cursor uneven, not all lines have the same thickness.

Without scaling everything looks fine:

[injust]

I don't use iTerm and haven't investigated if they do this, but this related idea seems like a reasonable approach:

I haven't seen this noted yet, but iTerm dynamically switches between (the equivalent of) font-thicken or not depending on the monitor, I believe by default; the preference for "Thin Strokes" supports "On Retina Displays" as an option. From some testing across a couple of monitors I have, I can replicate iTerm's rendering by manually toggling font-thicken depending on the monitor (after adjusting cell-width, but that's more just preference) - perhaps a font-thicken-retina/font-thicken-hidpi option might help?

Originally posted by @rmccue in #4504