element-web/res/css/views/messages/_ViewSourceEvent.scss

/*
Copyright 2019 The Matrix.org Foundation C.I.C.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

.mx_EventTile_content.mx_ViewSourceEvent {
    display: flex;
    opacity: 0.6;
    font-size: $font-12px;
    width: 100%;
    overflow-x: auto; // Cancel overflow setting of .mx_EventTile_content
    line-height: normal; // Align with avatar and E2E icon

    pre,
    code {
        flex: 1;
    }

    pre {
        line-height: 1.2;
        margin: 3.5px 0;
    }

    .mx_ViewSourceEvent_toggle {
        visibility: hidden;
        // override styles from AccessibleButton
        border-radius: 0;
        // icon
        mask-repeat: no-repeat;
        mask-position: 0 center;
        mask-size: auto 12px;
        width: 12px;
        background-color: $accent;
        mask-image: url("$(res)/img/element-icons/maximise-expand.svg");
    }

    &.mx_ViewSourceEvent_expanded .mx_ViewSourceEvent_toggle {
        mask-position: 0 bottom;
        margin-bottom: 5px;
        mask-image: url("$(res)/img/element-icons/minimise-collapse.svg");
    }
}

.mx_EventTile:hover {
    .mx_ViewSourceEvent_toggle {
        visibility: visible;
    }
}