From 52e46fa7a8b0e13836c8b12b23e3e8b631927470 Mon Sep 17 00:00:00 2001
From: Andros Fenollosa <hi@andros.dev>
Date: Wed, 3 Jun 2026 19:48:02 +0200
Subject: [PATCH] Metal: clear bottom strip when minibuffer height changes
 (expose substitute)

NS repaints uncovered regions through its drawRect: expose path; the
Metal overlay has none, so when a multi-line echo message shrank back,
stale pixels (e.g. a wrap-arrow shard in the echo fringe) survived.
Detect minibuffer pixel-height changes in update_begin and clear the
affected bottom strip inside the same update cycle, so the redraw and
the clear land in one present without flashing.
---
 src/mtlterm.h |  5 +++++
 src/mtlterm.m | 23 +++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/src/mtlterm.h b/src/mtlterm.h
index 949b64f5236..63732df5b55 100644
--- a/src/mtlterm.h
+++ b/src/mtlterm.h
@@ -174,6 +174,11 @@ typedef struct mtl_spring {
    the whole clear+redraw sequence is shown in one go (no flicker). */
 @property (nonatomic, assign) BOOL                        needsPresent;
 
+/* Expose substitute: last known pixel height of the minibuffer window.  When it
+   changes, update_begin clears the affected bottom strip (NS relies on its
+   drawRect: expose path for the uncovered pixels; Metal has none). */
+@property (nonatomic, assign) int                         lastMiniHeight;
+
 /* Main Emacs render cycle (renders to staticTexture) */
 - (void)beginFrame;
 - (void)endFrame;
diff --git a/src/mtlterm.m b/src/mtlterm.m
index a6c65f9039a..7e27220bab5 100644
--- a/src/mtlterm.m
+++ b/src/mtlterm.m
@@ -2703,6 +2703,29 @@ mtl_update_begin (struct frame *f)
      end it cleanly before starting a new frame. */
   if (fd.encoder) [fd endFrame];
   [fd beginFrame];
+
+  /* Expose substitute: when the minibuffer (echo area) changes height, the
+     bottom of the layout shifts but the engine does not repaint every
+     uncovered pixel — it relies on an expose pass that NS gets via drawRect:
+     and Metal does not have.  (Seen as a stale wrap-arrow shard in the echo
+     fringe after a multi-line message shrank back.)  Clear the affected
+     bottom strip at the start of this same update; the update then redraws
+     the real rows on top, all within one present, so nothing flashes. */
+  if (WINDOWP (FRAME_MINIBUF_WINDOW (f)))
+    {
+      struct window *mini = XWINDOW (FRAME_MINIBUF_WINDOW (f));
+      int mh = WINDOW_PIXEL_HEIGHT (mini);
+      if (fd.lastMiniHeight > 0 && mh != fd.lastMiniHeight)
+        {
+          int maxh = mh > fd.lastMiniHeight ? mh : fd.lastMiniHeight;
+          int y0 = FRAME_PIXEL_HEIGHT (f) - maxh - FRAME_LINE_HEIGHT (f);
+          if (y0 < 0) y0 = 0;
+          [fd fillRect:NSMakeRect (0, y0, FRAME_PIXEL_WIDTH (f),
+                                    FRAME_PIXEL_HEIGHT (f) - y0)
+                 color:ns_color_to_pixel (FRAME_BACKGROUND_COLOR (f))];
+        }
+      fd.lastMiniHeight = mh;
+    }
 }
 
 static void