Use the correct case in completion pager

Consider

	$ complete -c foo -a 'aab aaB' -f
	$ foo A<TAB>

since 28d67c8 we would insert the common prefix AND show the pager.
Due to case-insensitive comparison, "b/B" was considered to be part
of the prefix. Since the prefix is added to each pager item [1]
we get wrong results. Fix this by removing the insensitive comparison
between completions - I don't think it was of much use anyway.
Commandline tokens are still matched case-insensitively, this is
just about completions.

Test this by running interactive fish inside tmux (pexpect's terminal
emulation not have enough capabilities).  Also add tests for recent
interactive regressions #7526 and #7738.

Closes #3978

[1]: b38a23a would solve this differently by giving every pager item
its own prefix, but was reverted since it needs more fixes.

CHANGELOG.rst

@@ -15,6 +15,7 @@ Scripting improvements
 
 Interactive improvements
 -------------------------
+- When there are multiple completion candidates, fish inserts their shared prefix. This prefix was computed in a case-insensitive way, resulting in wrong case in the completion pager. This was fixed by only inserting prefixes with matching case (:issue:`7744`).
 
 New or improved bindings
 ^^^^^^^^^^^^^^^^^^^^^^^^

src/reader.cpp

@@ -1996,15 +1996,7 @@ bool reader_data_t::handle_completions(const completion_list_t &comp, size_t tok
                 size_t idx, max = std::min(common_prefix.size(), el.completion.size());
 
                 for (idx = 0; idx < max; idx++) {
-                    wchar_t ac = common_prefix.at(idx), bc = el.completion.at(idx);
-                    bool matches = (ac == bc);
-                    // If we are replacing the token, allow case to vary.
-                    if (will_replace_token && !matches) {
-                        // Hackish way to compare two strings in a case insensitive way,
-                        // hopefully better than towlower().
-                        matches = (wcsncasecmp(&ac, &bc, 1) == 0);
-                    }
-                    if (!matches) break;
+                    if (common_prefix.at(idx) != el.completion.at(idx)) break;
                 }
 
                 // idx is now the length of the new common prefix.

tests/checks/tmux-complete.fish

@@ -0,0 +1,49 @@
+#RUN: %fish -C 'set -g fish %fish' %s
+#REQUIRES: command -v tmux
+
+# Isolated tmux.
+set -g tmpdir (mktemp -d)
+set -g tmux tmux -S $tmpdir/.tmux-socket -f /dev/null
+
+set -g sleep sleep .3 # TSan tests in the CI failed with .1.
+
+set fish (realpath $fish)
+cd $tmpdir
+
+$tmux new-session -d $fish -C '
+    # This is similar to "tests/interactive.config".
+    function fish_greeting; end
+    function fish_prompt; printf "prompt $status_generation> "; end
+    # No autosuggestion from older history.
+    set fish_history ""
+'
+$tmux resize-window -x 80 -y 10
+$sleep # Let fish draw a prompt.
+
+# Don't escape existing token (#7526).
+echo >file-1
+echo >file-2
+$tmux send-keys 'HOME=$PWD ls ~/' Tab
+$sleep
+$tmux capture-pane -p
+# CHECK: prompt 0> HOME=$PWD ls ~/file-
+# CHECK: ~/file-1  ~/file-2
+
+# No pager on single smartcase completion (#7738).
+$tmux send-keys C-u C-l 'mkdir cmake CMakeFiles' Enter C-l \
+    'cat cmake' Tab
+$sleep
+$tmux capture-pane -p
+# CHECK: prompt 1> cat cmake/
+
+# Correct case in pager when prefies differ in case (#7743).
+$tmux send-keys C-u C-l 'complete -c foo2 -a "aabc aaBd" -f' Enter C-l \
+    'foo2 A' Tab
+$sleep
+$tmux capture-pane -p
+# The "bc" part is the autosuggestion - we could use "tmux capture-pane -e" to check colors.
+# CHECK: prompt 2> foo2 aabc
+# CHECK: aabc  aaBd
+
+$tmux kill-server
+rm -r $tmpdir