Early concept for rendering the frequency waveform

res/css/views/voice_messages/_FrequencyBars.scss

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+/*
+Copyright 2021 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_FrequencyBars {
+    position: relative;
+    height: 30px; // tallest bar can only be 30px
+
+    display: flex;
+    align-items: center; // so the bars grow from the middle
+
+    .mx_FrequencyBars_bar {
+        width: 2px;
+        margin-left: 1px;
+        margin-right: 1px;
+        background-color: $muted-fg-color;
+        display: inline-block;
+        min-height: 2px;
+        max-height: 100%;
+        border-radius: 2px; // give them soft endcaps
+    }
+}