compose: Fix reduced space below compose box after expanding it.

When expanding the compose box to full screen size, the buttons below
the compose box would unexpectedly jump because of how the 100% height
interacted with padding in the default box-sizing model.

Switching to border-box fixes this.

Fixes part of #19353.

static/styles/compose.css

@@ -68,6 +68,7 @@
     height: 100%;
     display: flex;
     flex-flow: column;
+    box-sizing: border-box;
 }
 
 .ztable_comp_col1 {