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diff --git a/TextViewport.hs b/TextViewport.hs
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+{-# LANGUAGE OverloadedStrings #-}
+
+module TextViewport
+  ( Item(..)
+  , Buffer(..)
+  , Viewport(..)
+  , RenderedLine(..)
+  , renderBuffer
+  , defaultViewport
+  , scrollUp
+  , scrollDown
+  , visibleLines
+  ) where
+
+import Data.Text (Text)
+import qualified Data.Text as T
+
+--------------------------------------------------------------------------------
+-- Logical model
+--------------------------------------------------------------------------------
+
+-- | A single buffer item. Arbitrary text, may contain hard line breaks.
+newtype Item = Item { unItem :: Text }
+
+-- | Oldest item first, newest last. No rendering assumptions here.
+newtype Buffer = Buffer { unBuffer :: [Item] }
+
+--------------------------------------------------------------------------------
+-- Rendered representation
+--------------------------------------------------------------------------------
+
+-- | A single line after splitting on hard breaks and cropping to viewport width.
+--   This is a *physical* line: no wrapping, only truncation.
+newtype RenderedLine = RenderedLine { unRenderedLine :: Text }
+  deriving (Eq, Show)
+
+-- | Render the entire buffer into a flat list of cropped lines.
+--   This is conceptually stable: scrolling operates only on this list.
+renderBuffer :: Int            -- ^ viewport width (characters)
+             -> Buffer
+             -> [RenderedLine]
+renderBuffer w (Buffer items) =
+    concatMap renderItem items
+  where
+    renderItem (Item t) =
+      let ls = T.splitOn "\n" t
+      in map (RenderedLine . crop) ls
+
+    crop = T.take w   -- hard truncation, no wrapping
+
+--------------------------------------------------------------------------------
+-- Viewport state
+--------------------------------------------------------------------------------
+
+-- | Viewport is defined by width, height, and a scroll offset into the
+--   rendered line stream. Offset is always a line index, never an item index.
+data Viewport = Viewport
+  { vpWidth  :: !Int
+  , vpHeight :: !Int
+  , vpOffset :: !Int   -- ^ index into rendered lines; 0 = top of buffer
+  } deriving (Eq, Show)
+
+-- | Construct a viewport positioned at the bottom (newest content).
+defaultViewport :: Int -> Int -> [RenderedLine] -> Viewport
+defaultViewport w h rendered =
+    let total = length rendered
+        off   = max 0 (total - h)
+    in Viewport w h off
+
+--------------------------------------------------------------------------------
+-- Scrolling
+--------------------------------------------------------------------------------
+
+-- | Scroll upward by k lines. Clamped at 0.
+scrollUp :: Int -> Viewport -> Viewport
+scrollUp k vp =
+    vp { vpOffset = max 0 (vpOffset vp - k) }
+
+-- | Scroll downward by k lines. Clamped at the last fully visible window.
+scrollDown :: Int -> [RenderedLine] -> Viewport -> Viewport
+scrollDown k rendered vp =
+    let total = length rendered
+        maxOff = max 0 (total - vpHeight vp)
+        newOff = min maxOff (vpOffset vp + k)
+    in vp { vpOffset = newOff }
+
+--------------------------------------------------------------------------------
+-- Visibility
+--------------------------------------------------------------------------------
+
+-- | Extract the currently visible slice of rendered lines.
+--   The viewport height determines the slice length.
+visibleLines :: [RenderedLine] -> Viewport -> [RenderedLine]
+visibleLines rendered vp =
+    take (vpHeight vp) . drop (vpOffset vp) $ rendered