Fix scrolling table with PageDown and PageUp

Resolves #402.

table.go

@@ -1101,27 +1101,37 @@ func (t *Table) InputHandler() func(event *tcell.EventKey, setFocus func(p Primi
 			}
 
 			pageDown = func() {
+				offsetAmount := t.visibleRows - t.fixedRows
+				if offsetAmount < 0 {
+					offsetAmount = 0
+				}
+
 				if t.rowsSelectable {
-					t.selectedRow += t.visibleRows
+					t.selectedRow += offsetAmount
 					if t.selectedRow >= len(t.cells) {
 						t.selectedRow = len(t.cells) - 1
 					}
 					next()
 				} else {
-					t.rowOffset += t.visibleRows
+					t.rowOffset += offsetAmount
 				}
 			}
 
 			pageUp = func() {
+				offsetAmount := t.visibleRows - t.fixedRows
+				if offsetAmount < 0 {
+					offsetAmount = 0
+				}
+
 				if t.rowsSelectable {
-					t.selectedRow -= t.visibleRows
+					t.selectedRow -= offsetAmount
 					if t.selectedRow < 0 {
 						t.selectedRow = 0
 					}
 					previous()
 				} else {
 					t.trackEnd = false
-					t.rowOffset -= t.visibleRows
+					t.rowOffset -= offsetAmount
 				}
 			}
 		)