Implement vertical cursor movement and reset cursor on file open

internal/editor/state.go

@@ -167,6 +167,7 @@ func GoToEditor(data any) {
 // data is the filename string from the browser list.
 func OpenFile(data any) {
 	TheState.Editor.Buffer = "Select a file to edit..." // Placeholder, should be loaded from file
+	TheState.Editor.CursorPosition = 0 // Reset cursor to top
 	TheState.ScrollOffset = 0 // Reset editor scroll to top when opening a new file
 	TheState.page = EditorPage
 	TheState.FocusedElementID = "editor_text" // Set focus to editor
@@ -214,6 +215,10 @@ func HandleKeyDown(data any) {
 			HandleCursorMove(-1)
 		case key.NameRightArrow:
 			HandleCursorMove(1)
+		case key.NameUpArrow:
+			HandleVerticalCursorMove(true)
+		case key.NameDownArrow:
+			HandleVerticalCursorMove(false)
 		case key.NameDeleteBackward:
 			HandleBackspace()
 		case key.NameDeleteForward:
@@ -222,6 +227,91 @@ func HandleKeyDown(data any) {
 	}
 }
 
+// HandleVerticalCursorMove updates the cursor position to the previous or next visual line.
+func HandleVerticalCursorMove(up bool) {
+	layout := TheState.Editor.GlyphLayout
+	if len(layout.ByteOffsets) == 0 {
+		return
+	}
+
+	pos := TheState.Editor.CursorPosition
+	// Find current glyph index.
+	idx := sort.Search(len(layout.ByteOffsets), func(i int) bool {
+		return layout.ByteOffsets[i] >= pos
+	})
+	// If idx == len, we are at the end. Use the last glyph.
+	if idx == len(layout.ByteOffsets) {
+		idx = len(layout.ByteOffsets) - 1
+	}
+
+	currentX := layout.X[idx]
+	currentY := layout.Y[idx]
+
+	var targetIdx int = idx
+
+	if up {
+		// Scan backwards to find the previous line's Y.
+		prevY := currentY
+		for i := idx; i >= 0; i-- {
+			if layout.Y[i] < prevY {
+				prevY = layout.Y[i]
+				break
+			}
+		}
+
+		if prevY == currentY {
+			// Already at the top line?
+			return
+		}
+
+		// Now find the closest X in prevY.
+		bestDiff := float32(1e9)
+		for i := 0; i < len(layout.Y); i++ {
+			if layout.Y[i] == prevY {
+				diff := float32(layout.X[i] - currentX)
+				if diff < 0 {
+					diff = -diff
+				}
+				if diff < bestDiff {
+					bestDiff = diff
+					targetIdx = i
+				}
+			}
+		}
+	} else {
+		// Scan forwards to find the next line's Y.
+		nextY := currentY
+		for i := idx; i < len(layout.Y); i++ {
+			if layout.Y[i] > nextY {
+				nextY = layout.Y[i]
+				break
+			}
+		}
+
+		if nextY == currentY {
+			// Already at the bottom line?
+			return
+		}
+
+		// Now find the closest X in nextY.
+		bestDiff := float32(1e9)
+		for i := 0; i < len(layout.Y); i++ {
+			if layout.Y[i] == nextY {
+				diff := float32(layout.X[i] - currentX)
+				if diff < 0 {
+					diff = -diff
+				}
+				if diff < bestDiff {
+					bestDiff = diff
+					targetIdx = i
+				}
+			}
+		}
+	}
+
+	TheState.Editor.CursorPosition = layout.ByteOffsets[targetIdx]
+}
+
 // HandleDelete removes the character after the cursor.
 func HandleDelete() {
 	pos := TheState.Editor.CursorPosition