Checked current main (a12d6ed942) using vp run dev in an isolated worktree on Linux, Chrome 154, DPR 2. Opened an empty thread, then a seeded long assistant message with headings, paragraphs, lists, file links, TypeScript and diff blocks. Scrolled and resized between 1280, 895 and 390 CSS pixels, with light and dark appearance. The overlapping text in your screenshot did not reproduce. Captures below show the seeded message at 895×759 and 390×844 in dark mode; these are web captures, not Electron or native mobile verification.
Please provide your T3 Code version, OS, whether this is the desktop app or a browser (and browser version), custom font/font size and zoom settings, and the affected message's Markdown with private content removed. Does reopening the thread or restarting the app clear it? Does it happen while the response streams, after scrolling, or only after resizing?