Force expanded panel display to 400px

Sets the CSS width for an expanded panel view to 400px in all cases.

Without this change, the feedback panel widths can oscillate between a width based
on the text in child panels and the maximum size of 400px when file operations
start and end so this locks the width to get rid of that oscillation.

Bug: 989154
Change-Id: I2902910b6e4436257f07e3b58c794c829d27cdac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735797
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684231}

ui/file_manager/file_manager/foreground/elements/xf_display_panel.js

@@ -107,12 +107,12 @@ class DisplayPanel extends HTMLElement {
                 }
                 75% {
                   max-height: calc(192px + 28px);
-                  max-width: 400px;
+                  width: 400px;
                   opacity: 0;
                 }
                 100% {
                   max-height: calc(192px + 28px);
-                  max-width: 400px;
+                  width: 400px;
                   opacity: 1;
                 }
               }
@@ -136,6 +136,7 @@ class DisplayPanel extends HTMLElement {
               }
               .expanded {
                 animation: setcollapse 200ms forwards;
+                width: 400px;
               }
               .collapsed {
                 animation: setexpand 200ms forwards;