From 5d6eaf606b5713365a232f188c922454943f2c09 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Thu, 7 Apr 2022 12:42:44 +0200
Subject: [PATCH] input: improve XTerm compatibility when modifyOtherKeys=2
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Before this, foot always emitted a CSI if _any_ modifier was
active. XTerm doesn’t behave quite like this. Instead, it appears to
special-case shift somewhat:

If the generated symbol (e.g ‘A’) is the upper case version of the
pressed key’s base symbol (‘a’), and is in the range a-z, emit a CSI.

If not emit a plain symbol:

Examples (Swedish layout):

* shift-a (generated symbol is ‘A’) emits a CSI
* shift-, (generated symbol is ‘;’) emits ‘;’
* shift-alt-, (generated symbol is ‘;’) emits a CSI

Closes #1009
---
 input.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/input.c b/input.c
index 1bf1e14b..ba45a59d 100644
--- a/input.c
+++ b/input.c
@@ -1096,7 +1096,56 @@ legacy_kbd_protocol(struct seat *seat, struct terminal *term,
     bool ctrl_is_in_effect = (keymap_mods & MOD_CTRL) != 0;
     bool ctrl_seq = is_control_key(sym) || (count == 1 && IS_CTRL(utf8[0]));
 
-    if (keymap_mods != MOD_NONE && (term->modify_other_keys_2 ||
+    bool modify_other_keys2_in_effect = false;
+
+    if (term->modify_other_keys_2) {
+        /*
+         * Try to mimic XTerm’s behavior, when holding shift:
+         *
+         *   - if other modifiers are pressed (e.g. Alt), emit a CSI escape
+         *   - upper-case symbols A-Z are encoded as an CSI escape
+         *   - other upper-case symbols (e.g ‘Ö’) or emitted as is
+         *   - non-upper cased symbols are _mostly_ emitted as is (foot
+         *     always emits as is)
+         *
+         * Examples (assuming Swedish layout):
+         *   - Shift-a (‘A’) emits a CSI
+         *   - Shift-, (‘;’) emits ‘;’
+         *   - Shift-Alt-, (Alt-;) emits a CSI
+         *   - Shift-ö (‘Ö’) emits ‘Ö’
+         */
+
+        /* Any modifiers, besides shift active? */
+        const xkb_mod_mask_t shift_mask = 1 << seat->kbd.mod_shift;
+        if ((ctx->mods & ~shift_mask & seat->kbd.bind_significant) != 0)
+            modify_other_keys2_in_effect = true;
+
+        else {
+            const xkb_layout_index_t layout_idx = xkb_state_key_get_layout(
+                seat->kbd.xkb_state, ctx->key);
+
+            /*
+             * Get pressed key’s base symbol.
+             *   - for ‘A’ (shift-a), that’s ‘a’
+             *   - for ‘;’ (shift-,), that’s ‘,’
+             */
+            const xkb_keysym_t *base_syms = NULL;
+            size_t base_count = xkb_keymap_key_get_syms_by_level(
+                seat->kbd.xkb_keymap, ctx->key, layout_idx, 0, &base_syms);
+
+            /* Check if base symbol(s) is a-z. If so, emit CSI */
+            const xkb_keysym_t lower_cased_sym = xkb_keysym_to_lower(ctx->sym);
+            for (size_t i = 0; i < base_count; i++) {
+                const xkb_keysym_t s = base_syms[i];
+                if (lower_cased_sym == s && s >= XKB_KEY_a && s <= XKB_KEY_z) {
+                    modify_other_keys2_in_effect = true;
+                    break;
+                }
+            }
+        }
+    }
+
+    if (keymap_mods != MOD_NONE && (modify_other_keys2_in_effect ||
                                     (ctrl_is_in_effect && !ctrl_seq)))
     {
         static const int mod_param_map[32] = {