editor: don't record a phantom trailing visual line at the window end

The shaper flags the last visible glyph of a final line without a
trailing newline (and the synthetic end-of-text glyph after a trailing
newline); the layout capture recorded both as visual-line starts at
len(str). Taps below the content mapped onto that phantom line
(harmless: it returned the EOF byte), but CaretPoint resolved the
caret through it and drew the caret on the phantom line - one line
BELOW where the typed text lands. Result: tap the empty space below
the last line, the cursor appears one line down, and the typed letter
appears one line ABOVE the cursor.

- render.go: only record a next-line start when a real line follows
  the break (a real newline cluster, or a soft wrap with content
  after it).
- state.go: a tap below every recorded line now returns the window
  end explicitly (the old clamp-to-last-text-line fallback remains
  only for layouts with no line starts at all).

Verified on the emulator for both 'test\nA' (caret now drawn at the
end of the 'A' line, where the letter lands) and 'test\nA\n' (caret
and letter on the trailing line); full test suite green.

internal/editor/state.go

@@ -3290,8 +3290,16 @@ func textPosOnLineAtX(visualLine int, x float64) (int, bool) {
 		if visualLine < len(layout.VisualLineStarts) {
 			return base + layout.VisualLineStarts[visualLine], true
 		}
-		// No line start recorded for this line (finger beyond the content):
-		// clamp to the last non-empty line.
+		// No line start recorded for this line (finger below the last line):
+		// the insertion point there is the end of the visible window — what
+		// the phantom trailing-line start used to provide (the shaper no
+		// longer records one at the window end; see the VisualLineStarts
+		// capture in drawWrappedText). A layout with NO line starts at all
+		// (degenerate/synthetic) has no window end to point at: clamp to
+		// the last non-empty line instead.
+		if len(layout.VisualLineStarts) > 0 {
+			return base + len(TheState.Editor.IMEWindowText), true
+		}
 		for i := len(groups) - 1; i >= 0; i-- {
 			if len(groups[i].indices) > 0 {
 				visualLine = i

internal/ui/render.go

@@ -1276,7 +1276,25 @@ func (r *Renderer) drawWrappedText(gtx layout.Context, str string, reg Region, w
 			flushLine()
 			if g.Flags&text.FlagLineBreak != 0 {
 				lineCount++
-				layout.VisualLineStarts = append(layout.VisualLineStarts, byteOffset)
+				// Record the next visual line's start ONLY when a real line
+				// follows the break: a real "\n" cluster (zero width, one
+				// rune — including the final one of a newline-terminated
+				// file, whose trailing empty line is real) or a soft wrap
+				// with content after it. The shaper ALSO flags the last
+				// visible glyph of a final line without a trailing newline,
+				// and the synthetic end-of-text glyph after a trailing
+				// newline: both sit at len(str) with no line after them.
+				// Recording them creates a phantom visual line: taps below
+				// the content map onto it (harmless — it returns the EOF
+				// byte), but CaretPoint resolves the caret through it and
+				// draws the caret on that phantom line — one line BELOW
+				// where the typed text lands, so the letter appeared a line
+				// above the cursor.
+				isRealNewline := g.Advance == 0 && g.Runes > 0
+				softWrap := g.Advance != 0 && byteOffset < len(str)
+				if isRealNewline || softWrap {
+					layout.VisualLineStarts = append(layout.VisualLineStarts, byteOffset)
+				}
 			}
 		}
 	}