diff --git a/web/src/buddy_list.ts b/web/src/buddy_list.ts
index 5195be1ed3..8e13604a61 100644
--- a/web/src/buddy_list.ts
+++ b/web/src/buddy_list.ts
@@ -12,7 +12,6 @@ import render_presence_rows from "../templates/presence_rows.hbs";
 import * as blueslip from "./blueslip.ts";
 import * as buddy_data from "./buddy_data.ts";
 import type {BuddyUserInfo} from "./buddy_data.ts";
-import {media_breakpoints_num} from "./css_variables.ts";
 import type {Filter} from "./filter.ts";
 import * as hash_util from "./hash_util.ts";
 import {$t} from "./i18n.ts";
@@ -26,6 +25,7 @@ import {current_user} from "./state_data.ts";
 import * as stream_data from "./stream_data.ts";
 import type {StreamSubscription} from "./sub_store.ts";
 import {INTERACTIVE_HOVER_DELAY} from "./tippyjs.ts";
+import * as ui_util from "./ui_util.ts";
 import {user_settings} from "./user_settings.ts";
 import * as util from "./util.ts";
 
@@ -174,7 +174,7 @@ export class BuddyList extends BuddyListConf {
                 e.stopPropagation();
                 const $elem = $(this);
                 let placement: "left" | "auto" = "left";
-                if (window.innerWidth < media_breakpoints_num.md) {
+                if (ui_util.matches_viewport_state("lt_md_min")) {
                     // On small devices display tooltips based on available space.
                     // This will default to "bottom" placement for this tooltip.
                     placement = "auto";
diff --git a/web/src/click_handlers.ts b/web/src/click_handlers.ts
index a11e335efa..d9ad291d3e 100644
--- a/web/src/click_handlers.ts
+++ b/web/src/click_handlers.ts
@@ -13,7 +13,6 @@ import * as buddy_data from "./buddy_data.ts";
 import * as compose_actions from "./compose_actions.ts";
 import * as compose_reply from "./compose_reply.ts";
 import * as compose_state from "./compose_state.ts";
-import {media_breakpoints_num} from "./css_variables.ts";
 import * as emoji_picker from "./emoji_picker.ts";
 import * as hash_util from "./hash_util.ts";
 import * as hashchange from "./hashchange.ts";
@@ -484,7 +483,7 @@ export function initialize(): void {
     ): void {
         let placement: tippy.Placement = "left";
         let observer: MutationObserver;
-        if (window.innerWidth < media_breakpoints_num.md) {
+        if (ui_util.matches_viewport_state("lt_md_min")) {
             // On small devices display tooltips based on available space.
             // This will default to "bottom" placement for this tooltip.
             placement = "auto";
diff --git a/web/src/popover_menus.ts b/web/src/popover_menus.ts
index 595567a34b..6bf7351040 100644
--- a/web/src/popover_menus.ts
+++ b/web/src/popover_menus.ts
@@ -6,11 +6,11 @@ import $ from "jquery";
 import * as tippy from "tippy.js";
 
 import * as blueslip from "./blueslip.ts";
-import {media_breakpoints_num} from "./css_variables.ts";
 import * as message_viewport from "./message_viewport.ts";
 import * as modals from "./modals.ts";
 import * as overlays from "./overlays.ts";
 import * as popovers from "./popovers.ts";
+import * as ui_util from "./ui_util.ts";
 import * as util from "./util.ts";
 
 type PopoverName =
@@ -418,7 +418,7 @@ export function toggle_popover_menu(
     // popover centered on the screen as an overlay.
     let show_as_overlay =
         (options?.show_as_overlay_on_mobile === true &&
-            window.innerWidth <= media_breakpoints_num.md) ||
+            ui_util.matches_viewport_state("lt_md_min")) ||
         options?.show_as_overlay_always === true;
     // Show the popover as over if the reference element is hidden.
     if (!show_as_overlay && options?.show_as_overlay_if_reference_hidden_at_trigger) {
diff --git a/web/src/sidebar_ui.ts b/web/src/sidebar_ui.ts
index dfd23be05f..04092fb78d 100644
--- a/web/src/sidebar_ui.ts
+++ b/web/src/sidebar_ui.ts
@@ -7,7 +7,6 @@ import render_right_sidebar from "../templates/right_sidebar.hbs";
 import {buddy_list} from "./buddy_list.ts";
 import * as channel from "./channel.ts";
 import * as compose_ui from "./compose_ui.ts";
-import {media_breakpoints_num} from "./css_variables.ts";
 import {reorder_left_sidebar_navigation_list} from "./left_sidebar_navigation_area.ts";
 import {localstorage} from "./localstorage.ts";
 import * as message_lists from "./message_lists.ts";
@@ -56,7 +55,7 @@ export function show_userlist_sidebar(): void {
         return;
     }
 
-    if (window.innerWidth >= media_breakpoints_num.xl) {
+    if (ui_util.matches_viewport_state("gte_xl_min")) {
         $("body").removeClass("hide-right-sidebar");
         fix_invite_user_button_flicker();
         return;
@@ -146,7 +145,7 @@ export function initialize(): void {
         e.preventDefault();
         e.stopPropagation();
 
-        if (window.innerWidth >= media_breakpoints_num.xl) {
+        if (ui_util.matches_viewport_state("gte_xl_min")) {
             $("body").toggleClass("hide-right-sidebar");
             if (!$("body").hasClass("hide-right-sidebar")) {
                 fix_invite_user_button_flicker();
@@ -172,11 +171,11 @@ export function initialize(): void {
         e.preventDefault();
         e.stopPropagation();
 
-        if (window.innerWidth >= media_breakpoints_num.md) {
+        if (ui_util.matches_viewport_state("gte_md_min")) {
             $("body").toggleClass("hide-left-sidebar");
             if (
                 message_lists.current !== undefined &&
-                window.innerWidth <= media_breakpoints_num.xl
+                !ui_util.matches_viewport_state("gte_xl_min")
             ) {
                 // We expand the middle column width between md and xl breakpoints when the
                 // left sidebar is hidden. This can cause the pointer to move out of view.
diff --git a/web/src/ui_util.ts b/web/src/ui_util.ts
index b7cddf4f65..a1c457b30a 100644
--- a/web/src/ui_util.ts
+++ b/web/src/ui_util.ts
@@ -258,3 +258,16 @@ export function show_left_sidebar_menu_icon(element: Element): void {
 export function hide_left_sidebar_menu_icon(): void {
     $(".left_sidebar_menu_icon_visible").removeClass("left_sidebar_menu_icon_visible");
 }
+
+export function matches_viewport_state(state_string: string): boolean {
+    const app_main = document.querySelector(".app-main");
+    if (app_main instanceof HTMLElement) {
+        const app_main_after_content = getComputedStyle(app_main, ":after").content ?? "";
+        /* The .content property includes the quotation marks, so we
+           strip them before splitting on the empty space. */
+        const app_main_after_content_array = app_main_after_content.replaceAll('"', "").split(" ");
+
+        return app_main_after_content_array.includes(state_string);
+    }
+    return false;
+}
diff --git a/web/styles/zulip.css b/web/styles/zulip.css
index 7c36236bf1..d5d9ba371b 100644
--- a/web/styles/zulip.css
+++ b/web/styles/zulip.css
@@ -495,6 +495,15 @@ body.has-overlay-scrollbar {
     height: 100%;
     min-height: 100%;
 
+    &::after {
+        /* We use `content` values to determine
+           viewport state from JavaScript; this
+           hides the values without disrupting
+           the document flow. */
+        position: absolute;
+        display: none;
+    }
+
     .column-left .left-sidebar,
     .column-right .right-sidebar {
         position: fixed;
@@ -1861,6 +1870,55 @@ body:not(.hide-left-sidebar) {
 
 /* End fallback media queries. */
 
+/* Begin viewport state queries. */
+
+/* We sometimes need the state of the viewport in JavaScript;
+   by setting values on `content:`, we can query instead for
+   those, independent of whether it's a container or media query
+   responsible for layout. See for example `sidebar_ui.ts`.
+
+   These queries for greater than or equal to should be ordered
+   smallest to largest. */
+
+/* Adjustments for smaller viewports are critical
+   regardless of info-density settings, so we
+   don't include a container-query check here. */
+@media (width < $md_min) {
+    .app-main::after {
+        content: "lt_md_min";
+    }
+}
+
+@container app (width >= $cq_md_min) {
+    .app-main::after {
+        content: "gte_md_min";
+    }
+}
+
+@media (width >= $md_min) {
+    .without-container-query-support {
+        .app-main::after {
+            content: "gte_md_min";
+        }
+    }
+}
+
+@container app (width >= $cq_xl_min) {
+    .app-main::after {
+        content: "gte_md_min gte_xl_min";
+    }
+}
+
+@media (width >= $xl_min) {
+    .without-container-query-support {
+        .app-main::after {
+            content: "gte_md_min gte_xl_min";
+        }
+    }
+}
+
+/* End viewport state queries. */
+
 @media (height < $short_navbar_cutoff_height) {
     .app-main .column-right.expanded .right-sidebar,
     .app-main .column-left.expanded .left-sidebar {