From ff3af3ac157a10c1b81c82c5c3d799a9f6c520e2 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Wed, 27 Sep 2023 19:54:56 +0530
Subject: [PATCH] E-book viewer: Make CFI calculation more robust especially on
 pages with very little content. Fixes #2037454 [E-book viewer: "Back" button
 doesn't work under some
 conditions](https://bugs.launchpad.net/calibre/+bug/2037454)

We were only testing two x values per y value. Instead test all 48.
---
 src/pyj/read_book/paged_mode.pyj | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/src/pyj/read_book/paged_mode.pyj b/src/pyj/read_book/paged_mode.pyj
index ce22ac66b7..0d242ed781 100644
--- a/src/pyj/read_book/paged_mode.pyj
+++ b/src/pyj/read_book/paged_mode.pyj
@@ -618,19 +618,20 @@ def current_cfi():
                 if yb <= top or ya >= bottom:
                     break
                 yidx += 1
-                xidx = 0
                 ys = v'[ya]' if ya is yb else v'[yb, ya]'
                 for cury in ys:
-                    xb, xa = midx - xidx * deltax, midx + xidx * deltax
-                    if xa <= left or xb >= right:
-                        break
-                    xidx += 1
-                    xs = v'[xa]' if xa is xb else v'[xb, xa]'
-                    for curx in xs:
-                        cfi = cfi_at_point(curx, cury)
-                        if cfi:
-                            # print('Viewport cfi:', cfi)
-                            return cfi
+                    xidx = 0
+                    while True:
+                        xb, xa = midx - xidx * deltax, midx + xidx * deltax
+                        if xb <= left or xa >= right:
+                            break
+                        xidx += 1
+                        xs = v'[xa]' if xa is xb else v'[xb, xa]'
+                        for curx in xs:
+                            cfi = cfi_at_point(curx, cury)
+                            if cfi:
+                                # print('Viewport cfi:', cfi)
+                                return cfi
     else:
         try:
             ans = cfi_at_current() or None