Resolve effective previewOpts before sizing input/list windows

hasInputWindow and the availableLines adjustment ran before
resizePreviewWindows, so they read t.activePreviewOpts from the previous
frame. When a size threshold flipped the layout to a posNext alternative
this frame, the input window was not reserved and the preview overlapped
the input row until the next redraw.

Walk the threshold chain up front to compute the effective previewOpts,
and base the input/availableLines decisions on it. Extract the
calculateSize boilerplate into a shared computePreviewSize helper.

src/terminal.go

@@ -2453,7 +2453,56 @@ func (t *Terminal) resizeWindows(forcePreview bool, redrawBorder bool) {
 	hasHeaderWindow := t.hasHeaderWindow()
 	hasFooterWindow := len(t.footer) > 0
 	hasHeaderLinesWindow, headerLinesShape := t.determineHeaderLinesShape()
-	hasInputWindow := !t.inputless && (t.inputBorderShape.Visible() || hasHeaderWindow || hasHeaderLinesWindow || t.activePreviewOpts.position == posNext)
+	// computePreviewSize returns the size resizePreviewWindows will compute
+	// for opts and the minimum size for that axis: height/minPreviewHeight
+	// for vertical positions, width/minPreviewWidth for horizontal.
+	computePreviewSize := func(opts *previewOpts) (int, int) {
+		minPreviewWidth, minPreviewHeight := t.minPreviewSize(opts)
+		switch opts.position {
+		case posUp, posDown, posNext:
+			minWindowHeight := minHeight
+			if t.inputless {
+				minWindowHeight--
+			}
+			if t.noSeparatorLine() {
+				minWindowHeight--
+			}
+			return calculateSize(height, opts.size, minWindowHeight, minPreviewHeight), minPreviewHeight
+		case posLeft, posRight:
+			minListWidth := minWidth
+			if t.listBorderShape.HasLeft() {
+				minListWidth += 2
+			}
+			if t.listBorderShape.HasRight() {
+				minListWidth++
+			}
+			return calculateSize(width, opts.size, minListWidth, minPreviewWidth), minPreviewWidth
+		}
+		return 0, 0
+	}
+	// Walk the threshold chain to determine the previewOpts that
+	// resizePreviewWindows will actually settle on. We need this here
+	// because hasInputWindow and the availableLines adjustment below run
+	// before resizePreviewWindows, and t.activePreviewOpts still holds the
+	// previous frame's resolution.
+	effectivePreviewOpts := &t.previewOpts
+	if t.needPreviewWindow() {
+		opts := &t.previewOpts
+		for {
+			if opts.size.size == 0 || opts.threshold <= 0 || opts.alternative == nil {
+				break
+			}
+			if actual, _ := computePreviewSize(opts); actual >= opts.threshold {
+				break
+			}
+			opts = opts.alternative
+			if opts.hidden {
+				break
+			}
+		}
+		effectivePreviewOpts = opts
+	}
+	hasInputWindow := !t.inputless && (t.inputBorderShape.Visible() || hasHeaderWindow || hasHeaderLinesWindow || effectivePreviewOpts.position == posNext)
 	inputWindowHeight := 2
 	if t.noSeparatorLine() {
 		inputWindowHeight--
@@ -2473,9 +2522,9 @@ func (t *Terminal) resizeWindows(forcePreview bool, redrawBorder bool) {
 
 	// FIXME: Needed?
 	if t.needPreviewWindow() {
-		_, minPreviewHeight := t.minPreviewSize(t.activePreviewOpts)
-		switch t.activePreviewOpts.position {
+		switch effectivePreviewOpts.position {
 		case posUp, posDown, posNext:
+			_, minPreviewHeight := t.minPreviewSize(effectivePreviewOpts)
 			availableLines -= minPreviewHeight
 		}
 	}
@@ -2703,20 +2752,12 @@ func (t *Terminal) resizeWindows(forcePreview bool, redrawBorder bool) {
 					t.pwindow.Erase()
 				}
 			}
-			minPreviewWidth, minPreviewHeight := t.minPreviewSize(previewOpts)
 			// Shared boilerplate for vertical positions (posUp/posDown/posNext):
 			// compute pheight, apply the threshold alternative, honor hidden,
 			// and update listStickToRight. Returns (pheight, true) when the
 			// caller should return early.
 			computeVerticalSize := func() (int, bool) {
-				minWindowHeight := minHeight
-				if t.inputless {
-					minWindowHeight--
-				}
-				if t.noSeparatorLine() {
-					minWindowHeight--
-				}
-				pheight := calculateSize(height, previewOpts.size, minWindowHeight, minPreviewHeight)
+				pheight, minPreviewHeight := computePreviewSize(previewOpts)
 				if hasThreshold && pheight < previewOpts.threshold {
 					t.activePreviewOpts = previewOpts.alternative
 					if forcePreview {
@@ -2783,14 +2824,7 @@ func (t *Terminal) resizeWindows(forcePreview bool, redrawBorder bool) {
 					createPreviewWindow(inputBorderTop()-pheight, marginInt[3], width, pheight)
 				}
 			case posLeft, posRight:
-				minListWidth := minWidth
-				if t.listBorderShape.HasLeft() {
-					minListWidth += 2
-				}
-				if t.listBorderShape.HasRight() {
-					minListWidth++
-				}
-				pwidth := calculateSize(width, previewOpts.size, minListWidth, minPreviewWidth)
+				pwidth, _ := computePreviewSize(previewOpts)
 				if hasThreshold && pwidth < previewOpts.threshold {
 					t.activePreviewOpts = previewOpts.alternative
 					if forcePreview {