fix(compositor): on Linux, the subtitle plate hugs the text

[EtienneLescot]

The bug

On Linux, subtitles get a huge amount of padding above and below in the editor preview.

The Linux backend drew a text block's background plate over the annotation's box (compositor_linux.rs, dst: anim_dst). For an ordinary annotation that mostly passes; for a subtitle, that box is the caption band — 22% of the frame height (CAPTION_BAND_HEIGHT_PCT) — so the fill extends far above and below the text.

Windows and macOS don't do this: since 889c068 they go through the shared text_plate box model (0.2em / 0.1em, radius in em) and the plate hugs the block. The Linux port never got that model — text_plate was there, Linux just didn't read it.

The fix

text_linux.rs:

• layout now runs on text_plate::layout_width (the box inset by the plate padding) instead of the full width — otherwise lines break somewhere other than where the plate is sized, and left/right-aligned text sticks to the edge;
• the rasterizer measures the block it actually laid out (line_w, line_top + line_height) and returns the plate rect alongside the atlas (TextAtlas / RasterizedGlyphs::plate): only it knows where cosmic-text placed the lines. Windows and macOS bake the plate into the texture; the R8 atlas here carries coverage only, hence the rect travelling separately.

compositor_linux.rs:

• the mode-1 quad uses that rect instead of the box quad, with text_plate::radius (in em, clamped by the plate) instead of the hardcoded 4.0 * rh / 1080;
• the plate is still revealed progressively by the typewriter animation, matching the backends that bake it into the texture.

Measurement

Caption band 1536x238, 48 px font, the text from the original screenshot:

	before	after
plate	1536 x 238	617 x 77

The ink occupies y101..142, the plate y80..157 — vertical slack drops from roughly 80 px on each side to ~20 px, of which 15 px is line height and 4.8 px is the 0.1em padding.

Tests

Two tests added in text_linux.rs:

• the_plate_hugs_the_text_instead_of_filling_the_box — in a deliberately over-tall box, the plate stays tight around the text and still covers all of it;
• the_plate_follows_the_alignment_and_stays_in_the_box — all three alignments put the plate on the correct side, never overflowing (beyond the box it would be cut off flat by the texture edge and lose its rounded corners).

⚠️ Not run on the machine that produced this patch: build.rs requires the ffmpeg headers, which aren't installed there, and there is no Linux Rust job in CI. The geometry was validated with a standalone harness replaying the CPU half of build_atlas against cosmic-text — the numbers above are its output. Please confirm with cargo test -p openscreen-compositor text_linux on a machine with the toolchain.

🤖 Generated with Claude Code

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