feat: add --page-overlap to keep N lines across a page scroll

[umputun]

pgdown and pgup advance the diff by a full screen, so the last line you read and the first line of the next screen share no context. --page-overlap N subtracts N from the page motion, carrying the bottom N rows to the top of the next screen.

default is 0, so paging is unchanged for anyone who does not set it. Also available as REVDIFF_PAGE_OVERLAP and page-overlap in the config file.

the whole behavior is a new pageRows() returning max(1, Height - overlap) where the two full-page motions previously passed viewport.Height directly. Everything else is config plumbing and docs.

three scope decisions worth stating:

• half-page motions are excluded. ctrl+d / ctrl+u already retain half a screen, so subtracting an overlap on top would shrink an already-overlapping motion. Pinned by a test.
• diff pane only. The file tree, markdown TOC and the two overlay pagers move by entry index rather than visual rows, where a row count has no meaning.
• a negative value clamps to 0 in NewModel rather than erroring, following --wrap-indent, which is the same shape of flag and clamps in the same place.

the carryover is approximate, not exact, and deviates both ways. Paging stops on cursor positions, and one position can span several rendered rows: a wrapped line or an annotation block at the page edge carries over more than N when the walk rolls back off it, and less than N when the walk takes it whole. In a plain unwrapped diff it is exactly N. This is stated in the pageRows godoc and in the README feature list; the flag description rows stay minimal per the project's flag-description rule.

Related to #311

[Copilot]

Pull request overview

Adds a configurable “page overlap” for full-page diff paging (PgUp/PgDn) so the next screen retains N rows of context from the previous screen, wired through CLI/env/config plus docs and tests.

Changes:

• Introduce --page-overlap N / REVDIFF_PAGE_OVERLAP / page-overlap config plumbing and documentation.
• Update diff paging to advance by max(1, viewportHeight - overlap) for PgUp/PgDn (half-page motions unchanged).
• Add tests covering config parsing, model config propagation, and paging behavior.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
site/docs.html	Documents the new --page-overlap option in the website options table.
README.md	Adds feature blurb and CLI options table entry for --page-overlap.
plugins/pi/skills/revdiff/SKILL.md	Adds examples demonstrating --page-overlap usage in the Pi skill docs.
plugins/codex/skills/revdiff/references/config.md	Documents --page-overlap in Codex skill config reference table.
.claude-plugin/skills/revdiff/references/config.md	Documents --page-overlap in Claude plugin config reference table.
app/config.go	Adds PageOverlap option flag/env/config definition.
app/config_test.go	Adds parsing precedence tests (default/flag/env/config/override) for PageOverlap.
app/main.go	Wires PageOverlap from parsed options into ui.ModelConfig.
app/ui/model.go	Adds pageOverlap mode state and ModelConfig.PageOverlap, clamping negative values.
app/ui/model_test.go	Tests NewModel propagation and negative clamping for PageOverlap.
app/ui/diffnav.go	Switches PgUp/PgDn to use pageRows() derived from viewport height minus overlap.
app/ui/diffnav_test.go	Adds behavioral tests for overlap on PgDn/PgUp and confirms half-page motions ignore overlap.

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