diff --git a/R/diffDashboard.R b/R/diffDashboard.R
index 2af2c6f..3511655 100644
--- a/R/diffDashboard.R
+++ b/R/diffDashboard.R
@@ -26,7 +26,7 @@ diffDashboard <- function(.file) {
   ui <- bslib::page_sidebar(
     title = htmltools::div(
       style = "display:flex; gap:.35rem; align-items:baseline; font-weight:600;",
-      htmltools::span("Visual diff:"),
+      htmltools::span("Revision Comparison:"),
       htmltools::span(
         style = "opacity:.7; font-weight:400;",
         fs::path_rel(.file)
@@ -104,10 +104,9 @@ diffDashboard <- function(.file) {
   server <- function(input, output, session) {
     session$onSessionEnded(function() shiny::stopApp())
 
-    # Selected IDs (strings): numeric revisions as strings + 'LOCAL'
-    # Default to newest vs LOCAL for immediate utility
+    # Unified selection state using extracted helpers
     default_sel <- c(as.character(newest), "Local")
-    sel <- shiny::reactiveVal(default_sel)
+    selection <- shiny::reactiveVal(compute_selection(default_sel))
 
     # show hint once the server has started
     show_app_exit_hint("diffDashboard")
@@ -115,54 +114,19 @@ diffDashboard <- function(.file) {
     shiny::observeEvent(
       input$rev_clicked,
       {
-        r <- as.character(input$rev_clicked)
-        cur <- sel()
-        if (r %in% cur) {
-          sel(setdiff(cur, r))
-        } else {
-          new <- c(cur, r)
-          new <- new[!duplicated(new)]
-          if (length(new) > 2) {
-            new <- utils::tail(new, 2)
-          }
-          sel(new)
-        }
+        new_ids <- update_selection(
+          selection()$ids,
+          input$rev_clicked,
+          max_sel = 2L
+        )
+        selection(compute_selection(new_ids))
       },
       ignoreInit = TRUE
     )
 
-    # Pairing logic: if LOCAL is present, force it to be `newer`
-    picked <- shiny::reactive({
-      x <- sel()
-      if (length(x) < 2) {
-        return(NULL)
-      }
-      if ("Local" %in% x) {
-        other <- setdiff(x, "Local")
-        if (!length(other)) {
-          return(NULL)
-        }
-        # choose the numeric rev as prior (use min in case of oddities)
-        other_num <- suppressWarnings(as.numeric(other))
-        other_num <- other_num[!is.na(other_num)]
-        if (!length(other_num)) {
-          return(NULL)
-        }
-        list(prior = min(other_num), newer = NULL)
-      } else {
-        # both numeric
-        nums <- suppressWarnings(as.numeric(x))
-        nums <- sort(nums)
-        if (length(nums) < 2) {
-          return(NULL)
-        }
-        list(prior = nums[1], newer = nums[2])
-      }
-    })
-
     # --- Timeline UI (LOCAL first, then SVN revisions) ---
     output$timeline_ui <- shiny::renderUI({
-      chosen <- sel()
+      chosen <- selection()$ids
 
       rev_items <- lapply(seq_len(nrow(svn_log)), function(i) {
         row <- svn_log[i, ]
@@ -195,8 +159,8 @@ diffDashboard <- function(.file) {
     })
 
     output$diff_html <- shiny::renderUI({
-      p <- picked()
-      shiny::req(p)
+      p <- selection()
+      shiny::req(!is.null(p$prior))
 
       sbs <- shiny::isTruthy(input$side_by_side)
       igw <- shiny::isTruthy(input$ignore_ws)
diff --git a/R/utils-dashboard.R b/R/utils-dashboard.R
new file mode 100644
index 0000000..13025ad
--- /dev/null
+++ b/R/utils-dashboard.R
@@ -0,0 +1,94 @@
+#' Update selected revision IDs based on a timeline click
+#'
+#' @description
+#' Toggle a clicked revision in the current selection, enforcing a maximum
+#' number of selections (default 2). Duplicates are removed while preserving
+#' first-occurrence order; when adding beyond the limit, the most recent
+#' selections are retained.
+#'
+#' @param ids `character()`
+#'   Current selected revision identifiers (e.g., `"Local"`, `"105"`).
+#' @param clicked `character(1)`
+#'   The revision identifier that was clicked.
+#' @param max_sel `integer(1)`
+#'   Maximum number of selections to retain. Defaults to `2L`.
+#'
+#' @return `character()` updated selection.
+#' @noRd
+update_selection <- function(ids, clicked, max_sel = 2L) {
+  clicked <- as.character(clicked)
+  ids <- as.character(ids)
+  
+  if (length(clicked) != 1L || is.na(clicked) || !nzchar(clicked)) {
+    return(ids)
+  }
+  
+  if (clicked %in% ids) {
+    new <- setdiff(ids, clicked)
+  } else {
+    new <- c(ids, clicked)
+    new <- new[!duplicated(new)]
+    if (length(new) > max_sel) new <- utils::tail(new, max_sel)
+  }
+  
+  new
+}
+
+#' Compute a paired selection (prior/newer) from selected revision IDs
+#'
+#' @description
+#' Given a character vector of selected revision identifiers (e.g., `"Local"`
+#' and/or numeric revisions as strings), compute the revision pair used for
+#' diffing. If `"Local"` is present, the local working copy is treated as the
+#' **newer** side and the minimum numeric revision among the other selections is
+#' treated as **prior**. If only numeric revisions are present, the two smallest
+#' numeric revisions are used as `prior` and `newer` respectively.
+#'
+#' @param ids `character()`
+#'   Selected revision identifiers. May include `"Local"` and/or numeric
+#'   revisions represented as character strings. Duplicates are ignored.
+#'
+#' @return `list` with elements:
+#' \describe{
+#'   \item{ids}{The de-duplicated input `ids` (character).}
+#'   \item{prior}{`numeric(1)` the prior revision number, or `NULL` if a pair
+#'     cannot be determined.}
+#'   \item{newer}{`numeric(1)` the newer revision number, or `NULL` to indicate
+#'     the local working copy when `"Local"` is selected.}
+#' }
+#'
+#' @details
+#' If fewer than two valid selections are provided, both `prior` and `newer`
+#' are `NULL`. Non-numeric entries other than `"Local"` are ignored when
+#' computing numeric revisions.
+#'
+#' @examples
+#' compute_selection(c("105", "Local"))
+#' compute_selection(c("101", "103"))
+#' compute_selection("Local")
+#'
+#' @noRd
+compute_selection <- function(ids) {
+  ids <- ids[!duplicated(ids)]
+  if (length(ids) < 2) {
+    return(list(ids = ids, prior = NULL, newer = NULL))
+  }
+  
+  if ("Local" %in% ids) {
+    other <- setdiff(ids, "Local")
+    other_num <- suppressWarnings(as.numeric(other))
+    other_num <- other_num[!is.na(other_num)]
+    if (!length(other_num)) {
+      return(list(ids = ids, prior = NULL, newer = NULL))
+    }
+    return(list(ids = ids, prior = min(other_num), newer = NULL))
+  } else {
+    nums <- suppressWarnings(as.numeric(ids))
+    nums <- sort(nums)
+    if (length(nums) < 2) {
+      return(list(ids = ids, prior = NULL, newer = NULL))
+    }
+    return(list(ids = ids, prior = nums[1], newer = nums[2]))
+  }
+}
+
diff --git a/pkgr.yml b/pkgr.yml
index 3da1013..b871b5e 100644
--- a/pkgr.yml
+++ b/pkgr.yml
@@ -11,9 +11,9 @@ Packages:
 
 Repos:
   - CRAN: https://cran.rstudio.com
-  - MPN: https://mpn.metworx.com/snapshots/stable/2024-06-12 # used for mrg packages
+  - MPN: https://mpn.metworx.com/snapshots/stable/2025-09-17 # used for mrg packages
 
 Lockfile:
     Type: renv
 
-Rpath: ${R_EXE_4_3}
\ No newline at end of file
+Rpath: ${R_EXE_4_5}
diff --git a/tests/testthat/test-selection.R b/tests/testthat/test-selection.R
new file mode 100644
index 0000000..788d3aa
--- /dev/null
+++ b/tests/testthat/test-selection.R
@@ -0,0 +1,129 @@
+# tests/testthat/test-selection.R
+
+testthat::test_that("update_selection: adds, toggles, de-duplicates, enforces max 2", {
+  # start empty, add one
+  ids <- character()
+  ids <- update_selection(ids, "105")
+  testthat::expect_identical(ids, c("105"))
+
+  # add second
+  ids <- update_selection(ids, "Local")
+  testthat::expect_identical(ids, c("105", "Local"))
+
+  # add third -> keeps the most recent two (tail)
+  ids <- update_selection(ids, "103")
+  testthat::expect_identical(ids, c("Local", "103"))
+
+  # clicking an existing id removes it (toggle off)
+  ids <- update_selection(ids, "Local")
+  testthat::expect_identical(ids, c("103"))
+
+  # adding a duplicate doesn't create another copy
+  ids <- update_selection(ids, "103")
+  testthat::expect_identical(ids, character())
+})
+
+testthat::test_that("update_selection: preserves first-occurrence order before truncation", {
+  ids <- c("Local", "110")
+  ids <- update_selection(ids, "105") # => c("Local","110","105") -> tail 2
+  testthat::expect_identical(ids, c("110", "105")) # last two kept in order they appeared
+})
+
+testthat::test_that("update_selection: accepts non-character & invalid clicks gracefully", {
+  # numeric click coerces to character
+  ids <- update_selection(character(), 105)
+  testthat::expect_identical(ids, "105")
+  testthat::expect_true(is.character(ids))
+
+  # empty string: no-op
+  ids2 <- update_selection(ids, "")
+  testthat::expect_identical(ids2, ids)
+
+  # NA: no-op
+  ids3 <- update_selection(ids, NA_character_)
+  testthat::expect_identical(ids3, ids)
+})
+
+testthat::test_that("update_selection: configurable max_sel works", {
+  ids <- character()
+  ids <- update_selection(ids, "101", max_sel = 3L)
+  ids <- update_selection(ids, "102", max_sel = 3L)
+  ids <- update_selection(ids, "103", max_sel = 3L)
+  testthat::expect_identical(ids, c("101", "102", "103"))
+
+  # adding a 4th keeps the last three
+  ids <- update_selection(ids, "104", max_sel = 3L)
+  testthat::expect_identical(ids, c("102", "103", "104"))
+})
+
+testthat::test_that("compute_selection: returns NULLs when fewer than two selections", {
+  s1 <- compute_selection(character())
+  testthat::expect_true(is.null(s1$prior))
+  testthat::expect_true(is.null(s1$newer))
+  testthat::expect_identical(s1$ids, character())
+
+  s2 <- compute_selection("Local")
+  testthat::expect_true(is.null(s2$prior))
+  testthat::expect_true(is.null(s2$newer))
+  testthat::expect_identical(s2$ids, "Local")
+
+  s3 <- compute_selection("abc")
+  testthat::expect_true(is.null(s3$prior))
+  testthat::expect_true(is.null(s3$newer))
+  testthat::expect_identical(s3$ids, "abc")
+})
+
+testthat::test_that("compute_selection: Local + one numeric => prior numeric, newer NULL", {
+  s <- compute_selection(c("105", "Local"))
+  testthat::expect_identical(s$prior, 105)
+  testthat::expect_true(is.null(s$newer))
+  testthat::expect_identical(s$ids, c("105", "Local"))
+})
+
+testthat::test_that("compute_selection: Local + multiple numerics => prior is min(other), newer NULL", {
+  s <- compute_selection(c("Local", "110", "105", "107"))
+  testthat::expect_identical(s$prior, 105)
+  testthat::expect_true(is.null(s$newer))
+  testthat::expect_identical(s$ids, c("Local", "110", "105", "107"))
+})
+
+testthat::test_that("compute_selection: two numerics => prior/newer are the two smallest", {
+  s <- compute_selection(c("110", "105"))
+  testthat::expect_identical(s$prior, 105)
+  testthat::expect_identical(s$newer, 110)
+  testthat::expect_identical(s$ids, c("110", "105"))
+})
+
+testthat::test_that("compute_selection: >2 numerics => uses the two smallest", {
+  s <- compute_selection(c("110", "105", "107", "200"))
+  testthat::expect_identical(s$prior, 105)
+  testthat::expect_identical(s$newer, 107)
+  testthat::expect_identical(s$ids, c("110", "105", "107", "200"))
+})
+
+testthat::test_that("compute_selection: ignores non-numeric strings (besides 'Local') for pairing", {
+  s <- compute_selection(c("foo", "105", "bar", "107"))
+  testthat::expect_identical(s$prior, 105)
+  testthat::expect_identical(s$newer, 107)
+  testthat::expect_identical(s$ids, c("foo", "105", "bar", "107"))
+})
+
+testthat::test_that("compute_selection: de-duplicates while preserving first occurrences", {
+  s <- compute_selection(c("Local", "105", "105", "Local", "107", "107"))
+  testthat::expect_identical(s$ids, c("Local", "105", "107"))
+  testthat::expect_identical(s$prior, 105)
+  testthat::expect_true(is.null(s$newer)) # Local present => newer is NULL
+})
+
+testthat::test_that("integration: update_selection + compute_selection behave as intended", {
+  ids <- character()
+  ids <- update_selection(ids, "105")
+  ids <- update_selection(ids, "Local")
+  # adding a third keeps last 2
+  ids <- update_selection(ids, "103")
+  testthat::expect_identical(ids, c("Local", "103"))
+
+  sel <- compute_selection(ids)
+  testthat::expect_identical(sel$prior, 103)
+  testthat::expect_true(is.null(sel$newer)) # Local implies newer is local
+})