vte: send escape sequence when ALT key is held

XTerm's altSendsEscape resource controls whether an escape character is
prepended to any output that is generated by keyboard input. We enable it
by default now.

Keyboard handling becomes kind of ugly now in the VTE layer. We should
definitely change this into some kind of lookup table or a more
sophisticated switch() handler in vte_input.c or similar.

Thanks to Tobias Wolf, "Etam" and Ran Benita for reporting this to the
bug-tracker and figuring out how this is correctly handled.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

src/vte.c

@@ -137,6 +137,7 @@ enum parser_action {
 #define FLAG_AUTO_REPEAT_MODE			0x00002000 /* Auto repeat key press; TODO: implement */
 #define FLAG_NATIONAL_CHARSET_MODE		0x00004000 /* Send keys from nation charsets; TODO: implement */
 #define FLAG_BACKGROUND_COLOR_ERASE_MODE	0x00008000 /* Set background color on erase (bce) */
+#define FLAG_PREPEND_ESCAPE			0x00010000 /* Prepend escape character to next output */
 
 struct vte_saved_state {
 	unsigned int cursor_x;
@@ -470,10 +471,17 @@ static void vte_write_debug(struct kmscon_vte *vte, const char *u8, size_t len,
 #endif
 
 	/* in local echo mode, directly parse the data again */
-	if (!vte->parse_cnt && !(vte->flags & FLAG_SEND_RECEIVE_MODE))
+	if (!vte->parse_cnt && !(vte->flags & FLAG_SEND_RECEIVE_MODE)) {
+		if (vte->flags & FLAG_PREPEND_ESCAPE)
+			kmscon_vte_input(vte, "\e", 1);
 		kmscon_vte_input(vte, u8, len);
+	}
 
+	if (vte->flags & FLAG_PREPEND_ESCAPE)
+		vte->write_cb(vte, "\e", 1, vte->data);
 	vte->write_cb(vte, u8, len, vte->data);
+
+	vte->flags &= ~FLAG_PREPEND_ESCAPE;
 }
 
 #define vte_write(_vte, _u8, _len) \
@@ -2124,6 +2132,16 @@ bool kmscon_vte_handle_keyboard(struct kmscon_vte *vte, uint32_t keysym,
 	size_t len;
 	const char *u8;
 
+	/* MOD1 (mostly labeled 'Alt') prepends an escape character to every
+	 * input that is sent by a key.
+	 * TODO: Transform this huge handler into a lookup table to save a lot
+	 * of code and make such modifiers easier to implement.
+	 * Also check whether altSendsEscape should be the default (xterm
+	 * disables this by default, why?) and whether we should implement the
+	 * fallback shifting that xterm does. */
+	if (mods & UTERM_MOD1_MASK)
+		vte->flags |= FLAG_PREPEND_ESCAPE;
+
 	if (mods & UTERM_CONTROL_MASK) {
 		switch (keysym) {
 		case XKB_KEY_2:
@@ -2561,5 +2579,6 @@ bool kmscon_vte_handle_keyboard(struct kmscon_vte *vte, uint32_t keysym,
 		return true;
 	}
 
+	vte->flags &= ~FLAG_PREPEND_ESCAPE;
 	return false;
 }