From 5318fc3ff38229f9a6fabcf0dbe02ea8620e7d2e Mon Sep 17 00:00:00 2001
From: Scott Bishel <scott.bishel@mattermost.com>
Date: Wed, 18 Feb 2026 18:02:13 -0700
Subject: [PATCH]   MM-66830: Fix RHS panel snapping to minimum width on resize
 (#35236)

Race condition caused mousemove events to fire after reset() but before
cleanup, resulting in negative width calculations that CSS clamped to
min-width.
---
 .../resizable_sidebar/resizable_divider.tsx   | 24 +++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/webapp/channels/src/components/resizable_sidebar/resizable_divider.tsx b/webapp/channels/src/components/resizable_sidebar/resizable_divider.tsx
index 349dc7c4033..b622c7cf9d0 100644
--- a/webapp/channels/src/components/resizable_sidebar/resizable_divider.tsx
+++ b/webapp/channels/src/components/resizable_sidebar/resizable_divider.tsx
@@ -129,7 +129,10 @@ function ResizableDivider({
             return;
         }
         previousClientX.current = e.clientX;
-        startWidth.current = containerRef.current.getBoundingClientRect().width;
+
+        const currentWidth = containerRef.current.getBoundingClientRect().width;
+        startWidth.current = currentWidth;
+        lastWidth.current = currentWidth;
 
         setIsActive(true);
 
@@ -160,7 +163,24 @@ function ResizableDivider({
 
             e.preventDefault();
 
-            const previousWidth = lastWidth.current ?? 0;
+            // Prevent race condition - if lastWidth is null, recover from container
+            // This can occur when a mousemove event fires after reset() but before cleanup
+            let previousWidth = lastWidth.current;
+            if (previousWidth === null || previousWidth === 0) {
+                const currentWidth = containerRef.current?.getBoundingClientRect().width ?? 0;
+                if (currentWidth > 0) {
+                    previousWidth = currentWidth;
+                    lastWidth.current = currentWidth;
+                    previousClientX.current = e.clientX;
+
+                    // Skip this mousemove, start fresh on next one
+                    return;
+                }
+
+                // If we can't determine width, return early to prevent negative width
+                return;
+            }
+
             let widthDiff = 0;
 
             switch (dir) {