diff --git a/src/main.tsx b/src/main.tsx
index 18ba55d0..1b20c8fc 100644
--- a/src/main.tsx
+++ b/src/main.tsx
@@ -12,7 +12,7 @@ import type {SessionDataType, LineType, CmdDataType, RemoteType, RemoteStateType
 import type * as T from "./types";
 import localizedFormat from 'dayjs/plugin/localizedFormat';
 import {GlobalModel, GlobalCommandRunner, Session, Cmd, ScreenLines, Screen, riToRPtr, windowWidthToCols, windowHeightToRows, termHeightFromRows, termWidthFromCols, TabColors, RemoteColors} from "./model";
-import {isModKeyPress} from "./util";
+import {isModKeyPress, boundInt} from "./util";
 import ReactMarkdown from 'react-markdown'
 import remarkGfm from 'remark-gfm'
 import {BookmarksView} from "./bookmarks";
@@ -1688,8 +1688,8 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
     lastOffsetHeight : number = 0;
     lastOffsetWidth : number = 0;
     ignoreNextScroll : boolean = false;
-    visibleMap : Map<string, OV<boolean>>;  // lineid => OV<vis>
-    collapsedMap : Map<string, OV<boolean>>;  // lineid => OV<collapsed>
+    visibleMap : Map<string, OV<boolean>>;  // linenum => OV<vis>
+    collapsedMap : Map<string, OV<boolean>>;  // linenum => OV<collapsed>
     lastLinesLength : number = 0;
     lastSelectedLine : number = 0;
 
@@ -1788,9 +1788,11 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
                 isVis = true
             }
             // console.log("line", lineElem.dataset.linenum, "top=" + lineTop, "bot=" + lineTop, isVis);
+            let lineNumInt = parseInt(lineElem.dataset.linenum);
             newMap.set(lineElem.dataset.linenum, isVis);
             // console.log("setvis", sprintf("%4d %4d-%4d (%4d) %s", lineElem.dataset.linenum, lineTop, lineBot, lineElem.offsetHeight, isVis));
         }
+        // console.log("compute vismap", "[" + this.firstVisLine + "," + this.lastVisLine + "]");
         mobx.action(() => {
             for (let [k, v] of newMap) {
                 let oldVal = this.visibleMap.get(k);
@@ -1824,31 +1826,29 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
     }
 
     restoreAnchorOffset(reason : string) : void {
-        let {screen} = this.props;
+        let {lines} = this.props;
         let linesElem = this.linesRef.current;
         if (linesElem == null) {
             return;
         }
-        if (screen.anchorLine == null || screen.anchorLine == 0) {
-            return;
-        }
-        let anchorElem = linesElem.querySelector(sprintf(".line[data-linenum=\"%d\"]", screen.anchorLine));
+        let anchor = this.getAnchor();
+        let anchorElem = linesElem.querySelector(sprintf(".line[data-linenum=\"%d\"]", anchor.anchorLine));
         if (anchorElem == null) {
             return;
         }
-        let isLastLine = screen.isLastLine(screen.anchorLine);
+        let isLastLine = (anchor.anchorIndex == lines.length-1);
         let scrollTop = linesElem.scrollTop;
         let height = linesElem.clientHeight;
         let containerBottom = scrollTop + height;
         let curAnchorOffset = containerBottom - (anchorElem.offsetTop + anchorElem.offsetHeight);
-        let newAnchorOffset = screen.anchorOffset;
+        let newAnchorOffset = anchor.anchorOffset;
         if (isLastLine && newAnchorOffset == 0) {
             newAnchorOffset = 10;
         }
         if (curAnchorOffset != newAnchorOffset) {
             let offsetDiff = curAnchorOffset - newAnchorOffset;
             let newScrollTop = scrollTop - offsetDiff;
-            // console.log("update scrolltop", reason, "line=" + screen.anchorLine, -offsetDiff, linesElem.scrollTop, "=>", newScrollTop);
+            // console.log("update scrolltop", reason, "line=" + anchor.anchorLine, -offsetDiff, linesElem.scrollTop, "=>", newScrollTop);
             linesElem.scrollTop = newScrollTop;
             this.ignoreNextScroll = true;
         }
@@ -1857,11 +1857,10 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
     componentDidMount() : void {
         let {screen, lines} = this.props;
         let linesElem = this.linesRef.current;
-        let anchorLineObj = screen.getLineByNum(screen.anchorLine);
-        if (anchorLineObj == null) {
-            // scroll to bottom
+        let anchor = this.getAnchor();
+        if (anchor.anchorIndex == lines.length-1) {
             if (linesElem != null) {
-                linesElem.scrollTop = linesElem.clientHeight;
+                linesElem.scrollTop = linesElem.scrollHeight;
             }
             this.computeAnchorLine();
         }
@@ -1870,7 +1869,6 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
         }
         this.lastSelectedLine = screen.getSelectedLine();
         this.lastLinesLength = lines.length;
-
         if (linesElem != null) {
             this.lastOffsetHeight = linesElem.offsetHeight;
             this.lastOffsetWidth = linesElem.offsetWidth;
@@ -1937,24 +1935,29 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
         if (newLine == 0) {
             return;
         }
+        let lidx = this.findClosestLineIndex(newLine);
         this.setLineVisible(newLine, true);
         // console.log("update selected line", this.lastSelectedLine, "=>", newLine, sprintf("anchor=%d:%d", screen.anchorLine, screen.anchorOffset));
         let viewInfo = this.getLineViewInfo(newLine);
+        let isFirst = (lidx.index == 0);
+        let isLast = (lidx.index == lines.length-1);
+        let offsetDelta = (isLast ? 10 : (isFirst ? -10 : 0));
         if (viewInfo == null) {
-            return;
+            screen.setAnchorFields(newLine, 0+offsetDelta, "updateSelectedLine");
         }
-        screen.setAnchorFields(newLine, viewInfo.anchorOffset, "updateSelectedLine");
-        let isFirst = (newLine == lines[0].linenum);
-        let isLast = (newLine == lines[lines.length-1].linenum);
-        if (viewInfo.botOffset > 0) {
-            linesElem.scrollTop = linesElem.scrollTop + viewInfo.botOffset + (isLast ? 10 : 0);
+        else if (viewInfo.botOffset > 0) {
+            linesElem.scrollTop = linesElem.scrollTop + viewInfo.botOffset + offsetDelta;
             this.ignoreNextScroll = true;
-            screen.anchorOffset = (isLast ? 10 : 0);
+            screen.setAnchorFields(newLine, offsetDelta, "updateSelectedLine");
         }
         else if (viewInfo.topOffset < 0) {
-            linesElem.scrollTop = linesElem.scrollTop + viewInfo.topOffset + (isFirst ? -10 : 0);
+            linesElem.scrollTop = linesElem.scrollTop + viewInfo.topOffset + offsetDelta;
             this.ignoreNextScroll = true;
-            screen.anchorOffset = linesElem.clientHeight - viewInfo.height;
+            let newOffset = linesElem.clientHeight - viewInfo.height;
+            screen.setAnchorFields(newLine, newOffset, "updateSelectedLine");
+        }
+        else {
+            screen.setAnchorFields(newLine, viewInfo.anchorOffset, "updateSelectedLine");
         }
         // console.log("new anchor", screen.getAnchorStr());
     }
@@ -2008,6 +2011,9 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
 
     @boundMethod
     onHeightChange(lineNum : number, newHeight : number, oldHeight : number) : void {
+        if (oldHeight == null) {
+            return;
+        }
         // console.log("height-change", lineNum, oldHeight, "=>", newHeight);
         this.restoreAnchorOffset("height-change");
         this.computeVisibleMap_debounced();
@@ -2032,6 +2038,38 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
         let prevLineFormat = dayjs(prevLineDate).format("YYYY-MM-DD");
         return null;
     }
+
+    findClosestLineIndex(lineNum : number) : {line : LineType, index : number} {
+        let {lines} = this.props;
+        if (lines.length == 0) {
+            throw new Error("invalid lines, cannot have 0 length in LinesView");
+        }
+        if (lineNum == null || lineNum == 0) {
+            return {line: lines[lines.length-1], index: lines.length-1};
+        }
+        // todo: bsearch
+        // lines is sorted by linenum
+        for (let idx=0; idx<lines.length; idx++) {
+            let line = lines[idx];
+            if (line.linenum >= lineNum) {
+                return {line: line, index: idx};
+            }
+        }
+        return {line: lines[lines.length-1], index: lines.length-1};
+    }
+
+    getAnchor() : {anchorLine : number, anchorOffset : number, anchorIndex : number} {
+        let {screen, lines} = this.props;
+        let anchor = screen.getAnchor();
+        if (anchor.anchorLine == null || anchor.anchorLine == 0) {
+            return {anchorLine: lines[lines.length-1].linenum, anchorOffset: 0, anchorIndex: lines.length-1};
+        }
+        let lidx = this.findClosestLineIndex(anchor.anchorLine);
+        if (lidx.line.linenum == anchor.anchorLine) {
+            return {anchorLine: anchor.anchorLine, anchorOffset: anchor.anchorOffset, anchorIndex: lidx.index};
+        }
+        return {anchorLine: lidx.line.linenum, anchorOffset: 0, anchorIndex: lidx.index};
+    }
     
     render() {
         let {screen, width, lines, renderMode} = this.props;
@@ -2052,7 +2090,11 @@ class LinesView extends React.Component<{sessionId : string, screen : Screen, wi
         let todayStr = getTodayStr();
         let yesterdayStr = getYesterdayStr();
         let prevDateStr : string = null;
-        for (let idx=0; idx<lines.length; idx++) {
+        let anchor = this.getAnchor();
+        let startIdx = boundInt(anchor.anchorIndex-50, 0, lines.length-1);
+        let endIdx = boundInt(anchor.anchorIndex+50, 0, lines.length-1);
+        // console.log("render", anchor, "[" + startIdx + "," + endIdx + "]");
+        for (let idx=startIdx; idx<=endIdx; idx++) {
             let line = lines[idx];
             let lineNumStr = String(line.linenum);
             let dateSepStr = null;
diff --git a/src/model.ts b/src/model.ts
index 9d3f67f5..ac7febf3 100644
--- a/src/model.ts
+++ b/src/model.ts
@@ -310,8 +310,7 @@ class Screen {
     lastRows : number;
     selectedLine : OV<number>;
     focusType : OV<FocusTypeStrs>;
-    anchorLine : number = null;
-    anchorOffset : number = 0;
+    anchor : OV<{anchorLine : number, anchorOffset : number}>;
     termLineNumFocus : OV<number>;
     setAnchor_debounced : (anchorLine : number, anchorOffset : number) => void;
     terminals : Record<string, TermWrap> = {};        // cmdid => TermWrap
@@ -328,7 +327,7 @@ class Screen {
         this.selectedLine = mobx.observable.box(sdata.selectedline == 0 ? null : sdata.selectedline, {name: "selectedLine"});
         this.setAnchor_debounced = debounce(1000, this.setAnchor.bind(this));
         if (sdata.selectedline != 0) {
-            this.setAnchorFields(sdata.selectedline, 0, "init");
+            this.anchor = mobx.observable.box({anchorLine: sdata.selectedline, anchorOffset: 0}, {name: "screen-anchor"});
         }
         this.termLineNumFocus = mobx.observable.box(0, {name: "termLineNumFocus"});
         this.curRemote = mobx.observable.box(sdata.curremote, {name: "screen-curRemote"});
@@ -368,10 +367,11 @@ class Screen {
     }
 
     getAnchorStr() : string {
-        if (this.anchorLine == null || this.anchorLine == 0) {
+        let anchor = this.anchor.get();
+        if (anchor.anchorLine == null || anchor.anchorLine == 0) {
             return "0";
         }
-        return sprintf("%d:%d", this.anchorLine, this.anchorOffset);
+        return sprintf("%d:%d", anchor.anchorLine, anchor.anchorOffset);
     }
 
     getTabColor() : string {
@@ -393,8 +393,9 @@ class Screen {
     }
 
     setAnchorFields(anchorLine : number, anchorOffset : number, reason : string) {
-        this.anchorLine = anchorLine;
-        this.anchorOffset = anchorOffset;
+        mobx.action(() => {
+            this.anchor.set({anchorLine: anchorLine, anchorOffset: anchorOffset});
+        })();
         // console.log("set-anchor-fields", anchorLine, anchorOffset, reason);
     }
 
@@ -435,6 +436,14 @@ class Screen {
         GlobalCommandRunner.screenSetAnchor(this.sessionId, this.screenId, setVal);
     }
 
+    getAnchor() : {anchorLine : number, anchorOffset : number} {
+        let anchor = this.anchor.get();
+        if (anchor.anchorLine == null || anchor.anchorLine == 0) {
+            return {anchorLine: this.selectedLine.get(), anchorOffset: 0};
+        }
+        return anchor;
+    }
+
     getMaxLineNum() : number {
         let win = this.getScreenLines();
         if (win == null) {
@@ -467,19 +476,6 @@ class Screen {
         return null;
     }
 
-    isLastLine(lineNum : number) : boolean {
-        let win = this.getScreenLines();
-        if (win == null) {
-            return false;
-        }
-        let lines = win.lines;
-        if (lines == null || lines.length == 0) {
-            return false;
-        }
-        let lastLine = lines[lines.length-1];
-        return (lastLine.linenum == lineNum);
-    }
-
     getPresentLineNum(lineNum : number) : number {
         let win = this.getScreenLines();
         if (win == null || !win.loaded.get()) {
@@ -2708,7 +2704,13 @@ class Model {
     runUpdate(genUpdate : UpdateMessage, interactive : boolean) {
         mobx.action(() => {
             let oldContext = this.getUIContext();
-            this.runUpdate_internal(genUpdate, oldContext, interactive);
+            try {
+                this.runUpdate_internal(genUpdate, oldContext, interactive);
+            }
+            catch (e) {
+                console.log("error running update", e, genUpdate);
+                throw e;
+            }
             let newContext = this.getUIContext()
             if (oldContext.sessionid != newContext.sessionid
                 || oldContext.screenid != newContext.screenid) {
diff --git a/src/util.ts b/src/util.ts
index 4c1f8818..2e7807c2 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -74,6 +74,10 @@ function genMergeSimpleData<T extends ISimpleDataType>(objs : mobx.IObservableAr
     }
     for (let i=0; i<dataArr.length; i++) {
         let dataItem = dataArr[i];
+        if (dataItem == null) {
+            console.log("genMergeSimpleData, null item");
+            console.trace();
+        }
         let id = idFn(dataItem);
         if (dataItem.remove) {
             delete objMap[id];
@@ -113,6 +117,11 @@ function genMergeData<ObjType extends IObjType<DataType>, DataType extends IData
     }
     for (let i=0; i<dataArr.length; i++) {
         let dataItem = dataArr[i];
+        if (dataItem == null) {
+            console.log("genMergeData, null item");
+            console.trace();
+            continue;
+        }
         let id = dataIdFn(dataItem);
         let obj = objMap[id];
         if (dataItem.remove) {
@@ -156,6 +165,11 @@ function genMergeDataMap<ObjType extends IObjType<DataType>, DataType extends ID
     }
     for (let i=0; i<dataArr.length; i++) {
         let dataItem = dataArr[i];
+        if (dataItem == null) {
+            console.log("genMergeDataMap, null item");
+            console.trace();
+            continue;
+        }
         let id = dataIdFn(dataItem);
         let obj = objMap.get(id);
         if (dataItem.remove) {