IME: fix post-commit caret for replacement commits; trace IME pushes

ApplyIMECommitToModel computed the caret pushed to the IME right
after a commit as Range.End + len(text). That is correct for a plain
insertion (Start == End) but wrong for a replacement — the case a
swipe/autocorrect over a selection produces: the inserted text starts
at Range.Start, so the caret is Range.Start + len(text). A 4-rune
replacement (the 'samet' autocorrect on the phone) pushed the caret
4 runes past the inserted text, desynchronizing the IME's caret from
ours at exactly the moment its local model was being updated; the
persistent 4-rune drift that followed (every commit 4 runes ahead,
then the empty-fix-up loop) matches this.

The logic side (applyIMECommitBytes) already computed the correct
caret; this aligns the renderer's immediate push with it.

Also log every IME push (snippet restarts, selection updates, commit
pushes) from the renderer: the phone incident's logcat rotated before
it could be re-analyzed, so the next occurrence must be
reconstructable from what we actually pushed to the IME.

internal/ui/render.go

@@ -24,6 +24,7 @@ import (
 	"gioui.org/unit"
 	"gioui.org/widget"
 	"golang.org/x/image/math/fixed"
+"log"
 )
 
 // maxInt32 is a large value used as MaxWidth for single-line text layout.
@@ -330,7 +331,15 @@ func (r *Renderer) ApplyIMECommitToModel(gtx layout.Context, ev key.EditEvent) {
 	r.imeHoldActive = true
 	r.imeHoldEditSeq = uint64(r.lastEditSeq)
 	r.imeHoldFrames = 0
-	caret := ev.Range.End + utf8.RuneCountInString(ev.Text)
+	// The post-commit caret is at the END of the inserted text, which
+	// starts at Range.Start: for an insertion (Start == End) that is
+	// Range.End + len, but for a replacement (a swipe/autocorrect over a
+	// selection) it is Range.Start + len — Range.End + len would land
+	// (End-Start) runes past the inserted text and desynchronize the IME's
+	// caret from ours (observed on the phone: a 4-rune replacement left
+	// every subsequent commit 4 runes ahead of the caret).
+	caret := ev.Range.Start + utf8.RuneCountInString(ev.Text)
+	log.Printf("IME PUSH commit-range=[%d,%d) text=%q -> sel caret=%d", ev.Range.Start, ev.Range.End, ev.Text, caret)
 	r.lastSelStart, r.lastSelCaret = -1, caret
 	r.lastIMECaretPos = r.imeCaretPos
 	gtx.Execute(key.SelectionCmd{
@@ -405,6 +414,7 @@ func (r *Renderer) FlushIME(gtx layout.Context, tf TextField) {
 		// this runs — see ApplyIMECommitToModel — so a normal keystroke
 		// never reaches this branch; it is the re-anchoring path: scroll,
 		// tap, file switch, snapped commit, external edit.)
+		log.Printf("IME PUSH snippet=[%d,%d) len=%d (restart)", snippet.Range.Start, snippet.Range.End, utf8.RuneCountInString(snippet.Text))
 		r.lastSelStart, r.lastSelCaret = -1, -1
 		gtx.Execute(key.SnippetCmd{Tag: tf.id, Snippet: snippet})
 	}
@@ -428,6 +438,7 @@ func (r *Renderer) FlushIME(gtx layout.Context, tf TextField) {
 			r.lastSelStart = selStart
 			r.lastSelCaret = selEnd
 			r.lastIMECaretPos = r.imeCaretPos
+			log.Printf("IME PUSH sel=[%d,%d)", selStart, selEnd)
 			rng := key.Range{Start: selStart, End: selEnd}
 			if selStart < 0 {
 				rng = key.Range{Start: selEnd, End: selEnd}