[kurtwindisch]

Summary

A partial fix for #1475. At a UI scale below 100% on macOS, white hairlines show around controls after they repaint. This PR hides the white and leaves the cause open. The options for a full fix are in #1475.

It also fixes the Film Strip scene marks, which the red selection outline covered.

Hairlines at a UI scale below 100%

• At a fractional device pixel ratio, a partial repaint on macOS leaves some device pixels unpainted. They show the NSWindow background, which AppKit makes light gray.
• Each shown window now takes THEME.bg_dark as its NSWindow background (negpy/desktop/view/mac_window.py). The gaps show that dark color instead of white.
• A translucent window, such as the tutorial overlay, keeps its clear background.
• The background is set once per native window. The app event filter passes each Show event, and the function checks the platform and that the widget is a top-level window.
• The photo canvas still draws at true screen pixels, and repaint cost does not change.

What it does not fix:

• The gaps are still there. On surfaces lighter than bg_dark, such as a card (#121212) or a header (#161616), a faint dark line can still show.
• The gap pixels show a few steps lighter than the color set (32 shows as 37 in a test window). The device color space gives the same result, so the sRGB color stays.
• An alternate color theme must update this fix. The module docstring and a comment at bg_dark in theme.py say so.

Scene marks on the Film Strip and the Light Table

• The selection outline painted over a scene's line and the bottom of its name chip. When the selected frame was the first of a scene, only a cut-off blue bar showed.
• The view now paints a scene's line across each run of its frames, then its name chip, after the selection outlines.
• The outline's two faded outer rings reach 1 to 2 pixels above the scene line, so a thin red edge still shows above it on a selected frame.

Tests

• tests/test_light_table.py: the scene line shows on a selected frame, and the outline still frames its sides.
• make all passes.
• Checked in the app on macOS at 90% UI scale, with a sandboxed user directory, a test negative and the Exposure tab:
  • The main window and a dialog read back #0D0D0D as their NSWindow background, and a translucent window is not changed.
  • After repeated slider updates, the stray pixels are dark (16 to 18), not white.
  • A slider update takes about 4 ms, as before.

Open questions

• The decisions in [Bug]: White hairlines around controls when UI scale is below 100% on macOS #1475 are still open: ship the downscale switch, or scale the UI in NegPy to remove the gaps.

🤖 Generated with Claude Code