(core) Keeps side panels closed when opening doc on a narrow screen

Test Plan: Includes nbrowser tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2709

app/client/ui2018/cssVars.ts

@@ -157,6 +157,10 @@ export const testId: TestId = makeTestId('test-');
 // Max width for narrow screen layout (in px). Note: 768px is bootstrap's definition of small screen
 export const maxNarrowScreenWidth = 768;
 
+export function isNarrowScreen() {
+  return window.innerWidth <= 768;
+}
+
 export const cssHideForNarrowScreen = styled('div', `
   @media (max-width: ${maxNarrowScreenWidth}px) {
     & {