diff --git a/pdl-live-react/src/view/masonry/MasonryCombo.tsx b/pdl-live-react/src/view/masonry/MasonryCombo.tsx
index ea6954f77..55570360c 100644
--- a/pdl-live-react/src/view/masonry/MasonryCombo.tsx
+++ b/pdl-live-react/src/view/masonry/MasonryCombo.tsx
@@ -11,7 +11,6 @@ import {
   Button,
   BackToTop,
   Modal,
-  type ModalProps,
   ModalBody,
   ModalFooter,
   ModalHeader,
@@ -28,6 +27,7 @@ import MasonryTileWrapper from "./MasonryTileWrapper"
 import Toolbar, { type SML } from "./Toolbar"
 
 import computeModel from "./model"
+import ConditionVariable from "./condvar"
 import {
   hasContextInformation,
   hasTimingInformation,
@@ -39,11 +39,7 @@ import RunningIcon from "@patternfly/react-icons/dist/esm/icons/running-icon"
 
 import "./Masonry.css"
 
-export type Runner = (
-  block: NonScalarPdlBlock,
-  onExit: () => void,
-  modalVariant?: ModalProps["variant"],
-) => void
+export type Runner = (block: NonScalarPdlBlock, onExit: () => void) => void
 
 type Props = {
   value: string
@@ -60,9 +56,6 @@ function setSMLUserSetting(sml: SML) {
 
 /** Combines <Masonry/>, <Timeline/>, ... */
 export default function MasonryCombo({ value, setValue }: Props) {
-  const [modalVariant, setModalVariant] =
-    useState<ModalProps["variant"]>("large")
-
   const block = useMemo(() => {
     if (value) {
       try {
@@ -83,11 +76,17 @@ export default function MasonryCombo({ value, setValue }: Props) {
     cmd: string
     args?: string[]
     onExit?: (exitCode: number) => void
+    cancelCondVar?: ConditionVariable
   }>(null)
+  const cancelModal = useCallback(
+    () => modalContent?.cancelCondVar?.signal(),
+    [modalContent?.cancelCondVar],
+  )
   const closeModal = useCallback(() => {
+    modalContent?.cancelCondVar?.signal()
     setModalContent(null)
     setModalIsDone(-1)
-  }, [setModalContent, setModalIsDone])
+  }, [modalContent?.cancelCondVar, setModalContent, setModalIsDone])
   const onExit = useCallback(
     (exitCode: number) => {
       setModalIsDone(exitCode)
@@ -97,19 +96,16 @@ export default function MasonryCombo({ value, setValue }: Props) {
     },
     [setModalIsDone, modalContent],
   )
+  useEffect(() => setModalIsDone(-2), [modalContent])
 
   // special form of setModalContent for running a PDL program
   const run = useCallback<Runner>(
-    async (runThisBlock, onExit, modalVariant) => {
+    async (runThisBlock, onExit) => {
       if (!isNonScalarPdlBlock(block)) {
         onExit()
         return
       }
 
-      if (modalVariant) {
-        setModalVariant(modalVariant)
-      }
-
       const [cmd, input, output] = (await invoke("replay_prep", {
         trace: JSON.stringify(runThisBlock),
         name:
@@ -132,6 +128,7 @@ export default function MasonryCombo({ value, setValue }: Props) {
           ...(!data ? [] : ["--data", JSON.stringify(data)]),
           input,
         ],
+        cancelCondVar: new ConditionVariable(),
         onExit: async () => {
           onExit()
           try {
@@ -198,15 +195,11 @@ export default function MasonryCombo({ value, setValue }: Props) {
 
       <BackToTop scrollableSelector=".pdl-masonry-page-section" />
 
-      <Modal
-        variant={modalVariant}
-        isOpen={!!modalContent}
-        onClose={closeModal}
-      >
+      <Modal variant="large" isOpen={!!modalContent} onClose={closeModal}>
         <ModalHeader
           title={modalContent?.header}
           titleIconVariant={
-            modalIsDone === -1
+            modalIsDone < 0
               ? RunningIcon
               : modalIsDone === 0
                 ? "success"
@@ -218,6 +211,7 @@ export default function MasonryCombo({ value, setValue }: Props) {
             <RunTerminal
               cmd={modalContent?.cmd ?? ""}
               args={modalContent?.args}
+              cancel={modalContent?.cancelCondVar}
               onExit={onExit}
             />
           </Suspense>
@@ -228,10 +222,18 @@ export default function MasonryCombo({ value, setValue }: Props) {
             key="Close"
             variant={modalIsDone > 0 ? "danger" : "primary"}
             onClick={closeModal}
-            isDisabled={modalIsDone === -1}
+            isDisabled={modalIsDone < 0}
           >
             Close
           </Button>
+          <Button
+            key="Cancel"
+            variant="secondary"
+            onClick={cancelModal}
+            isDisabled={modalIsDone !== -2}
+          >
+            Cancel
+          </Button>
         </ModalFooter>
       </Modal>
     </>
diff --git a/pdl-live-react/src/view/masonry/MasonryTile.tsx b/pdl-live-react/src/view/masonry/MasonryTile.tsx
index 223b75bc4..2bf949e34 100644
--- a/pdl-live-react/src/view/masonry/MasonryTile.tsx
+++ b/pdl-live-react/src/view/masonry/MasonryTile.tsx
@@ -53,7 +53,7 @@ export default function MasonryTile({
   const myRun = useCallback(() => {
     if (block && run) {
       setIsRunning(true)
-      run(block, () => setIsRunning(false), "medium")
+      run(block, () => setIsRunning(false))
     }
   }, [block, run, setIsRunning])
 
diff --git a/pdl-live-react/src/view/masonry/condvar.ts b/pdl-live-react/src/view/masonry/condvar.ts
new file mode 100644
index 000000000..61baf0a7d
--- /dev/null
+++ b/pdl-live-react/src/view/masonry/condvar.ts
@@ -0,0 +1,29 @@
+export default class ConditionVariable {
+  private condition: boolean = false
+  private waitingPromises: {
+    resolve: () => void
+    reject: (reason?: unknown) => void
+  }[] = []
+
+  public async wait(): Promise<void> {
+    if (this.condition) {
+      return Promise.resolve()
+    }
+
+    return new Promise((resolve, reject) => {
+      this.waitingPromises.push({ resolve, reject })
+    })
+  }
+
+  public signal(): void {
+    if (this.waitingPromises.length > 0) {
+      const { resolve } = this.waitingPromises.shift()!
+      resolve()
+      this.condition = true
+    }
+  }
+
+  public reset(): void {
+    this.condition = false
+  }
+}
diff --git a/pdl-live-react/src/view/term/RunTerminal.tsx b/pdl-live-react/src/view/term/RunTerminal.tsx
index fcba32da8..f6fb50983 100644
--- a/pdl-live-react/src/view/term/RunTerminal.tsx
+++ b/pdl-live-react/src/view/term/RunTerminal.tsx
@@ -8,16 +8,25 @@ import { ClipboardAddon } from "@xterm/addon-clipboard"
 import "./RunTerminal.css"
 
 type Props = {
+  /** The cmd part of `cmd ...args` */
   cmd: string
+
+  /** The args part of `cmd ...args */
   args?: string[]
+
+  /** A callback provided by caller to be invoked upon pty completion */
   onExit?: (exitCode: number) => void
+
+  /** A condition variable used by caller to request pty cancellation */
+  cancel?: import("../masonry/condvar").default
 }
 
-export default function RunTerminal({ cmd, args = [], onExit }: Props) {
+export default function RunTerminal({ cmd, args = [], onExit, cancel }: Props) {
   const ref = createRef<HTMLDivElement>()
   const [term, setTerm] = useState<null | Terminal>(null)
   const [exitCode, setExitCode] = useState(-1)
 
+  /** Schema adapter from our props.onExit to that of tauri-pty */
   const onExit2 = useCallback(
     ({ exitCode }: { exitCode: number }) => {
       setExitCode(exitCode)
@@ -28,7 +37,8 @@ export default function RunTerminal({ cmd, args = [], onExit }: Props) {
     [onExit, setExitCode],
   )
 
-  useEffect(() => setExitCode(-1), [cmd, args, onExit])
+  /** Re-initialization of exit code if props change */
+  useEffect(() => setExitCode(-1), [cmd, args, onExit, cancel])
 
   // Why a two-stage useEffect? Otherwise: cannot read properties of
   // undefined (reading 'dimensions')
@@ -64,6 +74,11 @@ export default function RunTerminal({ cmd, args = [], onExit }: Props) {
         rows: term.rows,
       })
 
+      /** Respond to cancellation request by killing the pty */
+      cancel?.wait().then(() => {
+        pty.kill()
+      })
+
       pty.onData((data) => term.write(data))
       term.onData((data) => pty.write(data))
 
@@ -78,7 +93,7 @@ export default function RunTerminal({ cmd, args = [], onExit }: Props) {
         }
       }
     }
-  }, [term, ref, exitCode, args, cmd, onExit2])
+  }, [term, ref, exitCode, cmd, args, cancel, onExit2])
 
   return (
     <div className="pdl-run-terminal" ref={ref} style={{ height: "600px" }} />