feat: configure half-page viewport scroll distance

docs/architecture.md

@@ -84,6 +84,8 @@ Resolution exposes its projected comparison through the same active DiffState an
 
 `Editor::scroll_distance()` supplies the session-wide optional edge margin. The TUI applies it to document/diff visual rows and unwrapped text columns, and retains independent picker offsets across navigation frames. Picker windows exclude their fixed chrome and align whole multi-row entries. File projections carry a monotonic viewport revision, so catalog/filter/expansion changes invalidate picker identity without hashing the entire catalog on every redraw. Metadata-only status changes do not move the viewport. Explicit LSP/wrap-toggle anchors and detached wheel views retain precedence.
 
+Keyboard viewport actions queue per-view row and half-page counts without moving carets. `Editor::take_viewport_scroll(height)` resolves half-page counts using the session-wide `scroll_half_page_lines` preference, or half the focused viewport when unset, and saturates multiplication before the frontend clamps movement to content bounds. Ordinary, diff, and conflict-history rendering share this resolution.
+
 Mouse wheel input uses the last rendered document/panel rectangles for hit-testing. Document offsets and explorer scroll state remain frontend-owned; three-row wheel steps never become document cursor motions. Manual document views persist until logical caret-reveal, caret/selection, mode, revision, or buffer changes; offscreen primary and browsing-selection cursors are not placed. Hover wheel input reuses the existing editor-owned popup scrolling.
 
 Already-ready mouse reports share a redraw: at most 64 reports are applied in their original order before painting, without dropping or summing away wheel steps. Readiness checks do not wait. The first non-mouse event is retained for the next iteration after rendering, so keys, paste, and resizes cannot reuse geometry from before their layout changes. The batch bound keeps continuous mouse input from starving drawing or background polling.

docs/configuration.md

@@ -16,6 +16,7 @@ Vamp.configure do |config|
   config.git_blame = true # false disables current-line attribution
   config.git_gutter = "bar" # or "symbols", "line_numbers", "none"
   config.scroll_distance = 0 # nonnegative edge distance, or nil to center
+  config.scroll_half_page_lines = nil # Ctrl+u/d: half viewport, or a positive screen-row count
   config.highlight_trailing_whitespace = false # true marks line-end whitespace in red
   config.whitespace_characters = {} # opt in: { " " => "·", "\t" => "→", "\n" => "↵" }
   config.whitespace_scope = "all" # or "leading" for indentation only
@@ -57,6 +58,8 @@ Plugin paths resolve relative to the config file, not the shell's working direct
 
 The setting applies to ordinary and historical buffers, split/unified diff panes, file pickers and explorers, command/buffer palettes, completion lists, and revision selectors. Wrapped text measures visual rows, not source lines; picker distances use result rows, excluding borders, search fields, and help. Manual mouse-wheel scrolling remains detached until keyboard navigation resumes. Only `nil` or integers from `0` through the platform's Rust `usize` maximum (`18446744073709551615` on 64-bit systems) are accepted; other values reject the configuration.
 
+`config.scroll_half_page_lines` controls Normal/Visual Ctrl+u/Ctrl+d (`core.scroll_half_up` / `core.scroll_half_down`). The default `nil` scrolls half the focused text viewport, rounded down with a minimum of one screen row. A positive integer, such as `config.scroll_half_page_lines = 3`, fixes the distance in screen rows, including wrapped continuations and diff display rows. Counts multiply that distance; `2` then Ctrl+d scrolls six rows with this example. This session-wide setting applies to ordinary, historical, diff, and conflict text panes without moving carets or selections. It does not change the cursor-edge margin, one-row scroll actions, mouse wheel, or picker navigation. Only `nil` or integers from `1` through the platform's Rust `usize` maximum are accepted; other values reject the configuration. Distances and count multiplication saturate at Rust's `isize` limit before the viewport clamps to content bounds.
+
 `config.highlight_trailing_whitespace` defaults to `false`. Set it to `true` to give trailing whitespace a red background in ordinary and historical buffers and both sides of split/unified diffs, including wrapped and horizontally scrolled text. Spaces, expanded tab cells, and Unicode whitespace at the end of a source line are highlighted; whitespace-only lines are included, but empty lines and padding beyond the text are not. Wrapping does not make interior whitespace trailing. Search, selections, and secondary carets retain their background precedence; diagnostic underlines remain visible. This changes display only, not file contents. Only literal Ruby booleans are accepted; other values reject the configuration.
 
 `config.whitespace_characters` defaults to `{}` (disabled). Assign a hash mapping literal whitespace characters to dim display markers, for example `{ " " => "·", "\t" => "→", "\u00a0" => "⍽", "\n" => "↵" }` for spaces, tabs, nonbreaking spaces, and line breaks. Unlisted characters keep their ordinary appearance. Keys and values must be single-character strings; keys must be whitespace with visible terminal width, tabs, or `"\n"`, and values must be printable one-cell characters. Carriage returns and other control characters, zero-width markers, and wide markers reject the configuration. Only indicator cells receive terminal dim styling; source-text colors, diagnostic underlines, and highlighted backgrounds remain intact.
@@ -208,7 +211,7 @@ Normal `C` / `D` / `Y` change/delete/copy the cursor-to-line-end suffix through
 
 `f<char>` / `F<char>` seeks forward/backward on the current line in Normal and Visual, with counts such as `2fx`. Rebind `config.core.seek_forward` / `config.core.seek_backward` to a single key or leader sequence; the action then waits for a literal target character. For example, `config.bind "s", config.core.seek_forward` makes `2sx` seek the second following `x`. Pass `mode: config.modes.visual` to install a Visual route. These native-only actions do not have Ex commands.
 
-Normal and Visual Ctrl+u/Ctrl+d invoke `config.core.scroll_half_up` / `config.core.scroll_half_down` without moving carets or selection endpoints. Counts multiply the half-viewport distance. Bind `config.core.scroll_up` / `config.core.scroll_down` for one screen row per count, for example `config.bind "ctrl+e", config.core.scroll_down` to replace clone-below in Normal. Pass `mode: config.modes.visual` to rebind Visual too. These actions work in ordinary and diff text panes, use wrapped screen rows, and have no Ex commands. Cursor navigation or editing resumes caret following.
+Normal and Visual Ctrl+u/Ctrl+d invoke `config.core.scroll_half_up` / `config.core.scroll_half_down` without moving carets or selection endpoints. Counts multiply the configured `scroll_half_page_lines` distance (half a viewport by default). Bind `config.core.scroll_up` / `config.core.scroll_down` for one screen row per count, for example `config.bind "ctrl+e", config.core.scroll_down` to replace clone-below in Normal. Pass `mode: config.modes.visual` to rebind Visual too. These actions work in ordinary and diff text panes, use wrapped screen rows, and have no Ex commands. Cursor navigation or editing resumes caret following.
 
 ## Action reference
 

docs/editing.md

@@ -31,7 +31,7 @@ Indicators refresh asynchronously. Typing and undo/redo retain the last complete
 | Normal, Visual | `<line>g` jumps immediately to that one-based source line at first nonblank (`42g`); clamps at EOF and extends Visual selections |
 | Normal | Ctrl+o goes to an older jump location; Ctrl+i/Tab goes forward; counts select multiple locations |
 | Normal, Visual | Decimal prefixes repeat cursor/word/search motions: `5j`, `3k`, `2w`, `4e`, `2n`; Esc cancels a pending count |
-| Normal, Visual | Ctrl+u/Ctrl+d scroll half a viewport up/down without moving carets or selections; counts multiply the distance |
+| Normal, Visual | Ctrl+u/Ctrl+d scroll up/down without moving carets or selections; half a viewport by default, configurable with `config.scroll_half_page_lines`; counts multiply the distance |
 | Normal | `J` (Shift+j) joins each targeted line with the next line |
 | Normal | `x`/Delete delete; `u` undo; `U` (Shift+u) redo; `:` enter command mode |
 | Normal | `yy` copies whole lines without moving; `dd` deletes them; `cc` changes them and enters Insert; counts work (`3yy`, `2dd`, `2cc`) |