fix: fill terminal canvas gap with measured padding

[jesse23]

Summary

• Fixes a permanent ~22px black bar on the right and bottom edges of the terminal
• Documents the file drag-and-drop path investigation as a rejected ADR

Fix: terminal canvas gap (ADR 022)

ghostty-web's `FitAddon` computes terminal columns as:

```
cols = floor((containerWidth - 15px scrollbar reserve) / charWidth)
```

Two factors leave the canvas short of the container edge every time:

1. Hardcoded 15px scrollbar reserve — `FitAddon` unconditionally subtracts 15px even when `overflow: hidden` means no scrollbar ever appears
2. Floor rounding — `Math.floor` discards the sub-cell remainder

The fix wraps `fitAddon.fit()` to measure the actual rendered canvas width after fitting, then distributes the gap as equal padding on each side — no CSS stretching, no blurriness. Also sets `container.background` from the theme config so any pre-render flash matches the terminal colour.

Upstream fix documented in ADR 022: add `IFitAddonOptions { scrollbarWidth?: number }` to ghostty-web's `FitAddon` constructor (no option exists today).

Docs: file DnD path analysis (ADR 021)

Records the full investigation into browser file drag-and-drop path injection — all five approaches evaluated, why none was adopted cross-platform, and reference to how code-server handles the same problem.

[Copilot]

Pull request overview

This PR addresses a persistent visual gap between the terminal container and the rendered Ghostty canvas by fitting the terminal and then distributing any remaining space as measured padding, and it adds ADR documentation for both the canvas-gap fix and a rejected file drag-and-drop path investigation.

Changes:

• Wraps FitAddon.fit() with a fit() helper that measures the actual canvas size and applies symmetric padding to eliminate the visible right/bottom gaps.
• Sets the terminal container background from the configured theme to avoid a mismatched pre-render flash.
• Adds ADR 022 (accepted) documenting the canvas-gap cause/workaround and ADR 021 (rejected) documenting the file DnD path constraints; adds an hn.md draft.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/client/index.ts	Replaces direct FitAddon resize handling with a measured-padding fit() wrapper and container ResizeObserver; sets container background from theme.
docs/hn.md	Adds a Show HN draft writeup.
docs/adrs/022.client.canvas-fill.md	Documents the root cause and the chosen workaround for the canvas gap, plus a proposed upstream fix.
docs/adrs/021.client.file-drop.md	Documents investigation and rejection rationale for file drag-and-drop path injection.

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