nextcloud-server/core/css/mobile.css

@media only screen and (max-width: 600px) {

/* compress search box on mobile, expand when focused */
.searchbox input[type="search"] {
	width: 17%;
	-webkit-transition: width 100ms;
	-moz-transition: width 100ms;
	-o-transition: width 100ms;
	transition: width 100ms;
}
.searchbox input[type="search"]:focus,
.searchbox input[type="search"]:active {
	width: 155px;
	-webkit-transition: width 100ms;
	-moz-transition: width 100ms;
	-o-transition: width 100ms;
	transition: width 100ms;
}

}