diff --git a/.agents/session-log.md b/.agents/session-log.md
index ac54d13..b047c40 100644
--- a/.agents/session-log.md
+++ b/.agents/session-log.md
@@ -385,5 +385,7 @@
 - 2026-04-08: Updated `.agents/workflow.md` so chat ownership is now a constant repo rule. The workflow file now defines stable Engine versus Desktop/UI ownership, cross-boundary stop rules, branch and dirty-tree preflight checks, and a requirement to record the validation actually run.
 - 2026-04-08: Started issue #214 renderer tab binding registry hardening. This slice is limited to `desktop/renderer/app.js` plus `.agents` continuity, and replaces the long `bindTabContentEvents` conditional chain with a local handler registry without changing renderer behavior.
 - 2026-04-08: Completed issue #214 renderer tab binding registry hardening. `bindTabContentEvents` now dispatches through a local `TAB_CONTENT_EVENT_BINDERS` registry in `desktop/renderer/app.js`, and `npm run smoke` still passes via local runs fallback.
+- 2026-04-10: Started issue #342 to restore primary workbench ownership after the desktop kept reserving too much width for empty or low-value side panes. This slice introduces a workbench-priority layout mode in the renderer so support and internal rails collapse earlier on common laptop widths instead of compressing the active surface.
+- 2026-04-10: Completed issue #342 primary workbench ownership. The renderer now marks workbench-heavy surfaces explicitly and collapses the outer support lane plus internal two-column workbench grids earlier, so `Runs`, `Compare`, `Candidates`, `Run Detail`, and `Paper Ops` stop holding two side rails at the same laptop-width breakpoint. Validation passed with `node --check desktop/renderer/app.js`, `npm run smoke:fallback`, and `npm run smoke:real-path`.
 - 2026-04-10: Started issue #346 to harden `desktop-smoke` result persistence after CI repeatedly failed with raw `ENOENT` on missing `result.json` in a planning-only PR. The slice is limited to `desktop/main.js`, `desktop/scripts/smoke.js`, and `.agents` continuity so smoke emits structured failures instead of crashing when Electron exits too early.
 - 2026-04-10: Added the desktop layout regression remediation block after reviewing the post-merge desktop state in real screenshots. Opened issues #342, #343, and #344 to target empty-pane collapse, stronger active-surface focus and context containment, and better runs-family density plus right-rail space budgeting without reopening core or `research_ui` scope.
diff --git a/desktop/renderer/app.js b/desktop/renderer/app.js
index 2641819..1bb6d27 100644
--- a/desktop/renderer/app.js
+++ b/desktop/renderer/app.js
@@ -105,6 +105,19 @@ const state = {
   workspacePersistTimer: null,
 };
 
+const WORKBENCH_PRIORITY_KINDS = new Set([
+  "runs",
+  "run",
+  "compare",
+  "candidates",
+  "artifacts",
+  "paper",
+  "system",
+  "experiments",
+  "sweep-decision",
+  "job",
+]);
+
 const elements = {
   runtimeSummary: document.getElementById("runtime-summary"),
   runtimeMeta: document.getElementById("runtime-meta"),
@@ -151,6 +164,7 @@ const elements = {
   workflowRunsList: document.getElementById("workflow-runs-list"),
   workflowOpenCompare: document.getElementById("workflow-open-compare"),
   workflowClearSelection: document.getElementById("workflow-clear-selection"),
+  workspaceGrid: document.querySelector(".workspace-grid"),
 };
 
 const paletteActions = PALETTE_ACTION_SPECS.map((action) => ({
@@ -909,6 +923,7 @@ function renderChatAdapterStatus() {
 
 function renderTabs() {
   if (!state.tabs.length) {
+    syncWorkspaceLayoutMode(null);
     clearElement(elements.tabsBar);
     appendChildren(
       elements.tabContent,
@@ -944,6 +959,7 @@ function renderTabs() {
   );
   const activeTab = state.tabs.find((tab) => tab.id === state.activeTabId) || state.tabs[0];
   state.activeTabId = activeTab.id;
+  syncWorkspaceLayoutMode(activeTab);
   elements.topbarTitle.textContent = activeTab.title;
   syncTopbarChrome(activeTab);
   syncNav(activeTab.navKind || activeTab.kind);
@@ -982,6 +998,14 @@ function renderTabs() {
   bindTabContentEvents(activeTab);
 }
 
+function syncWorkspaceLayoutMode(activeTab) {
+  if (!elements.workspaceGrid) return;
+  const kind = activeTab?.kind || "";
+  const priority = WORKBENCH_PRIORITY_KINDS.has(kind) ? "high" : "normal";
+  elements.workspaceGrid.dataset.activeTabKind = kind;
+  elements.workspaceGrid.dataset.workbenchPriority = priority;
+}
+
 function renderPalette() {
   elements.paletteOverlay.classList.toggle("hidden", !state.paletteOpen);
   if (!state.paletteOpen) return;
diff --git a/desktop/renderer/styles.css b/desktop/renderer/styles.css
index 7726d68..5ef9263 100644
--- a/desktop/renderer/styles.css
+++ b/desktop/renderer/styles.css
@@ -409,7 +409,7 @@ button:disabled {
 .tabs-shell {
     display: grid;
     grid-template-rows: auto auto 1fr;
-    min-height: 500px;
+    min-height: 420px;
     gap: 4px;
 }
 
@@ -1572,6 +1572,35 @@ button:disabled {
     line-height: 1.55;
 }
 
+@media (max-width: 1560px) {
+    .workspace-grid[data-workbench-priority="high"] {
+        grid-template-columns: 1fr;
+    }
+
+    .workspace-grid[data-workbench-priority="high"] .support-column {
+        order: 2;
+    }
+
+    .workspace-grid[data-workbench-priority="high"] .tabs-shell {
+        min-height: 0;
+    }
+
+    .workspace-grid[data-workbench-priority="high"] .runs-workbench,
+    .workspace-grid[data-workbench-priority="high"] .compare-workbench,
+    .workspace-grid[data-workbench-priority="high"] .candidates-workbench,
+    .workspace-grid[data-workbench-priority="high"] .run-detail-grid,
+    .workspace-grid[data-workbench-priority="high"] .run-detail-evidence-grid,
+    .workspace-grid[data-workbench-priority="high"] .evidence-grid {
+        grid-template-columns: 1fr;
+    }
+
+    .workspace-grid[data-workbench-priority="high"] .runs-workbench-side,
+    .workspace-grid[data-workbench-priority="high"] .run-detail-side {
+        position: static;
+        top: auto;
+    }
+}
+
 @media (max-width: 1200px) {
     .workspace-grid { grid-template-columns: 1fr; }
     .workflow-grid { grid-template-columns: 1fr; }