Keep latestYBase fresh once scrollback is full

Recording the live bottom only when yDisp moved worked until the
scrollback filled up: from then on Terminal.scroll recycles lines
instead of advancing yBase, so yDisp stops changing and any stale
value — from a resize, a font change, or a spell in scrollback —
was never corrected, leaving status detection reading the wrong row.

Snapshot yDisp on every chunk that arrives while the viewport is at
the bottom instead, which is the one moment yDisp equals yBase. That
also removes the manual scrollTo restore: SwiftTerm already keeps the
viewport put when the user is browsing scrollback.

Author: bones7456

Notchy/TerminalManager.swift

@@ -22,13 +22,17 @@ class ClickThroughTerminalView: LocalProcessTerminalView {
     private var pendingURL: URL?
     private var pendingCWD: String?
 
-    // SwiftTerm forces yDisp back to yBase on every line scroll because its
-    // `userScrolling` flag is never set from the scroll wheel path. We track
-    // the live bottom (yBase) ourselves: after super.dataReceived runs, if
-    // yDisp moved we know it now equals yBase, so we record it. Status
-    // detection reads from this row so it sees the latest output even while
-    // the user is browsing scrollback; the dataReceived override also uses
-    // it to restore the user's manual scroll position after each chunk.
+    // The buffer row of the live bottom (SwiftTerm's `yBase`, which it keeps
+    // internal). Status detection reads the grid from this row so it sees the
+    // latest output even while the user is browsing scrollback.
+    //
+    // We snapshot it from `yDisp` on every chunk that arrives while the
+    // viewport is at the bottom — the one moment the two are equal. Recording
+    // on *every* such chunk (rather than only when yDisp moved) matters: once
+    // the scrollback fills up, `Terminal.scroll` stops advancing yBase and
+    // recycles lines instead, so yDisp never changes again and a value that
+    // went stale — from a resize, a font change, or a spell in scrollback —
+    // would otherwise never be corrected.
     private var latestYBase: Int = 0
 
     // SwiftTerm's NSTextInputClient implementation drops marked (preedit)
@@ -691,36 +695,29 @@ class ClickThroughTerminalView: LocalProcessTerminalView {
         allowMouseReporting = terminal.mouseMode != .off
 
         let wasAlternate = terminal.isCurrentBufferAlternate
-        let preYDisp = terminal.buffer.yDisp
-        // Only treat the viewport as "in scrollback" on the normal buffer.
-        // The alternate buffer (vim/less) has no scrollback and its yDisp is
-        // unrelated to latestYBase, so comparing them would spuriously fire.
-        let wasInScrollback = !wasAlternate && preYDisp < latestYBase
 
         super.dataReceived(slice: slice)
         hasNewData = true
 
         // Re-read the buffer: super may have switched buffers (entering or
-        // leaving vim via \e[?1049h/l). yDisp isn't comparable across that
-        // switch, so skip all scroll bookkeeping unless we stayed on the
-        // normal buffer for the whole chunk. Without this guard, leaving the
-        // alternate screen runs scrollTo on the freshly-restored normal
-        // buffer and snaps it to the top.
+        // leaving vim via \e[?1049h/l). The alternate buffer's yDisp is
+        // unrelated to the normal buffer's live bottom, so only record when we
+        // stayed on the normal buffer for the whole chunk.
         guard !wasAlternate, !terminal.isCurrentBufferAlternate else { return }
 
-        // Snapshot the new yBase so extractAllLines can read the live bottom
-        // even when the viewport is parked in scrollback.
-        if terminal.buffer.yDisp != preYDisp {
+        if isViewportAtBottom {
             latestYBase = terminal.buffer.yDisp
         }
+    }
 
-        // Only restore the viewport if the user was already browsing
-        // scrollback before this chunk arrived. When the user is at the
-        // bottom (preYDisp == old latestYBase), let SwiftTerm's auto-scroll
-        // keep them there.
-        if wasInScrollback {
-            scrollTo(row: preYDisp, notifyAccessibility: false)
-        }
+    /// Whether the viewport is following the live bottom rather than parked in
+    /// scrollback — the only moment `yDisp` is guaranteed to equal `yBase`.
+    ///
+    /// `scrollPosition` reports 1 once `yDisp` reaches the last scrollback row,
+    /// and `canScroll` is false when the whole buffer fits on screen, which
+    /// makes the viewport trivially "at the bottom".
+    private var isViewportAtBottom: Bool {
+        !canScroll || scrollPosition >= 1
     }
 
     private func evaluateStatus(for id: UUID) {