implement nano-like page up/page down functionality

internal/action/actions.go

@@ -1669,63 +1669,77 @@ func (h *BufPane) End() bool {
 
 // PageUp scrolls the view up a page
 func (h *BufPane) PageUp() bool {
-	h.ScrollUp(h.BufView().Height)
+	pageOverlap := int(h.Buf.Settings["pageoverlap"].(float64))
+	h.ScrollUp(h.BufView().Height - pageOverlap)
 	return true
 }
 
 // PageDown scrolls the view down a page
 func (h *BufPane) PageDown() bool {
-	h.ScrollDown(h.BufView().Height)
+	pageOverlap := int(h.Buf.Settings["pageoverlap"].(float64))
+	h.ScrollDown(h.BufView().Height - pageOverlap)
 	h.ScrollAdjust()
 	return true
 }
 
 // SelectPageUp selects up one page
 func (h *BufPane) SelectPageUp() bool {
+	pageOverlap := int(h.Buf.Settings["pageoverlap"].(float64))
+	scrollAmount := h.BufView().Height - pageOverlap
 	if !h.Cursor.HasSelection() {
 		h.Cursor.OrigSelection[0] = h.Cursor.Loc
 	}
-	h.MoveCursorUp(h.BufView().Height)
+	h.MoveCursorUp(scrollAmount)
 	h.Cursor.SelectTo(h.Cursor.Loc)
+	if h.Cursor.Num == 0 {
+		h.ScrollUp(scrollAmount)
+	}
 	h.Relocate()
 	return true
 }
 
 // SelectPageDown selects down one page
 func (h *BufPane) SelectPageDown() bool {
+	pageOverlap := int(h.Buf.Settings["pageoverlap"].(float64))
+	scrollAmount := h.BufView().Height - pageOverlap
 	if !h.Cursor.HasSelection() {
 		h.Cursor.OrigSelection[0] = h.Cursor.Loc
 	}
-	h.MoveCursorDown(h.BufView().Height)
+	h.MoveCursorDown(scrollAmount)
 	h.Cursor.SelectTo(h.Cursor.Loc)
+	if h.Cursor.Num == 0 {
+		h.ScrollDown(scrollAmount)
+		h.ScrollAdjust()
+	}
 	h.Relocate()
 	return true
 }
 
-// CursorPageUp places the cursor a page up
+// CursorPageUp places the cursor a page up,
+// moving the view to keep cursor at the same relative position in the view
 func (h *BufPane) CursorPageUp() bool {
 	h.Cursor.Deselect(true)
-
-	if h.Cursor.HasSelection() {
-		h.Cursor.Loc = h.Cursor.CurSelection[0]
-		h.Cursor.ResetSelection()
-		h.Cursor.StoreVisualX()
+	pageOverlap := int(h.Buf.Settings["pageoverlap"].(float64))
+	scrollAmount := h.BufView().Height - pageOverlap
+	h.MoveCursorUp(scrollAmount)
+	if h.Cursor.Num == 0 {
+		h.ScrollUp(scrollAmount)
 	}
-	h.MoveCursorUp(h.BufView().Height)
 	h.Relocate()
 	return true
 }
 
-// CursorPageDown places the cursor a page up
+// CursorPageDown places the cursor a page down,
+// moving the view to keep cursor at the same relative position in the view
 func (h *BufPane) CursorPageDown() bool {
 	h.Cursor.Deselect(false)
-
-	if h.Cursor.HasSelection() {
-		h.Cursor.Loc = h.Cursor.CurSelection[1]
-		h.Cursor.ResetSelection()
-		h.Cursor.StoreVisualX()
+	pageOverlap := int(h.Buf.Settings["pageoverlap"].(float64))
+	scrollAmount := h.BufView().Height - pageOverlap
+	h.MoveCursorDown(scrollAmount)
+	if h.Cursor.Num == 0 {
+		h.ScrollDown(scrollAmount)
+		h.ScrollAdjust()
 	}
-	h.MoveCursorDown(h.BufView().Height)
 	h.Relocate()
 	return true
 }

internal/config/settings.go

@@ -32,6 +32,7 @@ var optionValidators = map[string]optionValidator{
 	"helpsplit":       validateChoice,
 	"matchbracestyle": validateChoice,
 	"multiopen":       validateChoice,
+	"pageoverlap":     validateNonNegativeValue,
 	"reload":          validateChoice,
 	"scrollmargin":    validateNonNegativeValue,
 	"scrollspeed":     validateNonNegativeValue,
@@ -76,6 +77,7 @@ var defaultCommonSettings = map[string]interface{}{
 	"matchbraceleft":  true,
 	"matchbracestyle": "underline",
 	"mkparents":       false,
+	"pageoverlap":     float64(2),
 	"permbackup":      false,
 	"readonly":        false,
 	"reload":          "prompt",

internal/display/softwrap.go

@@ -291,13 +291,7 @@ func (w *BufWindow) diff(s1, s2 SLoc) int {
 // within the buffer boundaries.
 func (w *BufWindow) Scroll(s SLoc, n int) SLoc {
 	if !w.Buf.Settings["softwrap"].(bool) {
-		s.Line += n
-		if s.Line < 0 {
-			s.Line = 0
-		}
-		if s.Line > w.Buf.LinesNum()-1 {
-			s.Line = w.Buf.LinesNum() - 1
-		}
+		s.Line = util.Clamp(s.Line + n, 0, w.Buf.LinesNum()-1)
 		return s
 	}
 	return w.scroll(s, n)

runtime/help/options.md

@@ -285,6 +285,13 @@ Here are the available options:
 
     default value: `tab`
 
+* `pageoverlap`: the number of lines from the current view to keep in view
+   when paging up or down. If this is set to 2, for instance, and you page
+   down, the last two lines of the previous page will be the first two lines
+   of the next page.
+
+    default value: `2`
+
 * `paste`: treat characters sent from the terminal in a single chunk as a paste
    event rather than a series of manual key presses. If you are pasting using
    the terminal keybinding (not `Ctrl-v`, which is micro's default paste