v0.6.0 mouse: every wheel direction and side button named; tui: a scrollbar the pointer drives

* mouse: the wheel's four directions and the side buttons

parseSGRMouse read a report's low two bits alone, so a wheel tilted left or
right (66, 67) reported as WheelDown, and a mouse's back and forward buttons
(128, 129) as a left or middle press. It now decodes by bank: bits 0-1 number
a button within the main three and none, the wheel's four directions (bit 6),
or buttons 8 to 11 (bit 7). WheelLeft and WheelRight join MouseButton after
Forward, so every existing value keeps its number; Back and Forward are
reported for buttons 8 and 9; buttons 10 and 11, and a report with both bank
bits set, are dropped as KeyNone. A wheel step is a press with no release in
every direction.

* tui: ScrollThumb and ScrollDrag

ScrollThumb is the thumb ScrollBarStyled draws, its first row and height,
exported so an application hit-tests the bar it drew. ScrollDrag drives a bar
by the pointer, rows counted from the track's top: a press on the thumb takes
hold where it lands, a press on the track moves the thumb's middle there and
takes hold, and each drag keeps the held row under the pointer, clamped at the
track's ends, at the nearest offset when there are fewer offsets than rows.
Each call returns the offset to show.

A table of SGR reports covers each bank and drops the buttons past 9; a
property test over every track of 3 to 10 rows and content up to 30 checks
each press and drag against the thumb ScrollThumb draws.

input.go

@@ -623,7 +623,8 @@ func (r *inputReader) parseControl(b byte) Event {
 }
 
 // parseSGRMouse parses mouse SGR sequences, ESC [ < Btn ; X ; Y M/m, from
-// the parameters and final byte; a malformed one is dropped as KeyNone
+// the parameters and final byte; a malformed one, or a button past 9, is
+// dropped as KeyNone
 func parseSGRMouse(params []byte, final byte) Event {
 	btn, x, y, ok := parseSGRParams(params)
 	if !ok {
@@ -632,48 +633,34 @@ func parseSGRMouse(params []byte, final byte) Event {
 
 	ev := Event{Type: EventMouse, MouseX: x - 1, MouseY: y - 1} // Convert to 0-indexed
 
-	// Decode button and action
-	// Bits 0-1: button (0=left, 1=middle, 2=right, 3=release)
-	// Bit 5 (32): motion
-	// Bit 6 (64): scroll
-	buttonID := btn & 0x03
-	isMotion := btn&32 != 0
-	isScroll := btn&64 != 0
+	// Bits 0-1 number a button within its bank: the main three and none, the
+	// wheel's four directions (bit 6), or buttons 8 to 11 (bit 7); bit 5 is motion
+	id := btn & 0x03
+	switch btn & (64 | 128) {
+	case 0:
+		ev.MouseBtn = [...]MouseButton{MouseBtnLeft, MouseBtnMiddle, MouseBtnRight, MouseBtnNone}[id]
+	case 64:
+		ev.MouseBtn = [...]MouseButton{MouseBtnWheelUp, MouseBtnWheelDown, MouseBtnWheelLeft, MouseBtnWheelRight}[id]
+	case 128:
+		if id > 1 {
+			return Event{Type: EventKey, Key: KeyNone}
+		}
+		ev.MouseBtn = MouseBtnBack + MouseButton(id)
+	default:
+		return Event{Type: EventKey, Key: KeyNone}
+	}
 
-	if isScroll {
-		// Scroll: buttonID 0=up, 1=down
-		if buttonID == 0 {
-			ev.MouseBtn = MouseBtnWheelUp
-		} else {
-			ev.MouseBtn = MouseBtnWheelDown
-		}
-		ev.MouseAction = MouseActionPress // Scroll is instantaneous
-	} else {
-		// Regular button
-		switch buttonID {
-		case 0:
-			ev.MouseBtn = MouseBtnLeft
-		case 1:
-			ev.MouseBtn = MouseBtnMiddle
-		case 2:
-			ev.MouseBtn = MouseBtnRight
-		case 3:
-			ev.MouseBtn = MouseBtnNone // Release with no specific button
-		}
-
-		if final == 'M' {
-			if isMotion {
-				if ev.MouseBtn != MouseBtnNone {
-					ev.MouseAction = MouseActionDrag
-				} else {
-					ev.MouseAction = MouseActionMove
-				}
-			} else {
-				ev.MouseAction = MouseActionPress
-			}
-		} else {
-			ev.MouseAction = MouseActionRelease
-		}
+	switch {
+	case btn&64 != 0:
+		ev.MouseAction = MouseActionPress // A wheel step is its press
+	case final == 'm':
+		ev.MouseAction = MouseActionRelease
+	case btn&32 == 0:
+		ev.MouseAction = MouseActionPress
+	case ev.MouseBtn != MouseBtnNone:
+		ev.MouseAction = MouseActionDrag
+	default:
+		ev.MouseAction = MouseActionMove
 	}
 
 	// Extract modifiers from button byte