audit: M+L tier polish (DDL UI batch 5-7)

Eight smaller items from docs/audit-2026-04-29-app-deep.md:

M-1: Off-grid margins snapped to 6/12/18 on the 6px GNOME grid:
  - structure_tab column / index / FK row vertical padding 2 → 6
    (three places, matches the rest of the boxed-list rows)
  - history_dialog pinned + all-queries heading bottom 4 → 6
  - sidebar schema-header top 8 / bottom 4 → 12 / 6 (anchors the
    schema label and the "+" button on grid).

M-5: WelcomeView's hard `set_size_request(560, -1)` replaced with
  ScrolledWindow `max_content_width(560) + propagate_natural_width`.
  Narrow viewports now collapse the welcome content gracefully
  instead of pushing past the window frame.

M-12: on_schema_changed actually uses the (schema, table) params.
  Iterates open Browse tabs and dispatches FetchBrowseColumns +
  FetchBrowsePage + FetchBrowseRowCount for every match. Previously
  the params were silently dropped — Browse tabs pointing at a
  table that just got altered showed stale columns until a manual
  F5. Now they refresh automatically alongside the sidebar.

L-1: editor_tab_tooltip switched from `q.chars().count()` (O(n) on
  the full query) to a `char_indices().take(200)` byte-prefix walk.
  For multi-megabyte SQL dumps we no longer scan past the 200-char
  preview boundary just to compute the truncate-vs-don't decision.

L-5: SQL Preview SourceView in the Structure tab now connects to
  AdwStyleManager::dark-notify and applies "Adwaita" / "Adwaita-dark"
  scheme dynamically — mirroring editor.rs. Previously the scheme
  was frozen at the boot theme even after the user toggled system
  dark mode.

Skipped as N/A:
- M-2 (sidebar row double-padding) — `.navigation-sidebar` row
  styling is theme-dependent; the inner 6px isn't necessarily
  doubled.
- M-4 (status page icon convention) — Browse loading uses spinner
  child, Structure loading uses synchronizing icon. Different
  affordances for different states; intentional.
- M-15 (Test button placement) — current placement matches GNOME
  Connections per existing inline comment.
- L-6 (keyboard menu set_pointing_to) — already correct via the
  present_row_menu(None) path landed in the lazy-popover commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: datlechin

linux/crates/app/src/ui/app/mod.rs

@@ -911,8 +911,8 @@ impl SimpleComponent for App {
             let header_box = gtk::Box::builder()
                 .orientation(gtk::Orientation::Horizontal)
                 .spacing(6)
-                .margin_top(8)
-                .margin_bottom(4)
+                .margin_top(12)
+                .margin_bottom(6)
                 .margin_start(12)
                 .margin_end(6)
                 .build();

linux/crates/app/src/ui/app/structure.rs

@@ -417,13 +417,36 @@ impl App {
     }
 
     /// Schema state changed somewhere — reload the table list, then
-    /// refetch any open Browse tab pointing at the affected table.
+    /// refetch any open Browse tab pointing at the affected table so
+    /// its grid reflects post-DDL schema (column adds / drops / type
+    /// changes / renames).
     pub(super) fn on_schema_changed(
         &self,
-        _schema: Option<String>,
-        _table: Option<String>,
+        schema: Option<String>,
+        table: Option<String>,
         sender: ComponentSender<Self>,
     ) {
+        // Refetch the affected Browse tab(s) immediately. Tab-id
+        // collection happens under a short-lived borrow; the sender
+        // dispatches happen after drop so the input handlers can
+        // re-borrow workspace_tabs without panic.
+        if let Some(table_name) = table.as_deref() {
+            let mut affected: Vec<Uuid> = Vec::new();
+            for (id, tab) in self.workspace_tabs.borrow().iter() {
+                if let WorkspaceTab::Browse(slot) = tab
+                    && slot.schema.as_deref() == schema.as_deref()
+                    && slot.table == table_name
+                {
+                    affected.push(*id);
+                }
+            }
+            for id in affected {
+                sender.input(AppMsg::FetchBrowseColumns(id));
+                sender.input(AppMsg::FetchBrowsePage(id));
+                sender.input(AppMsg::FetchBrowseRowCount(id));
+            }
+        }
+        // Sidebar refresh: re-list tables and rebuild the factory.
         let sender_for_cmd = sender.clone();
         sender.command(move |_, shutdown| {
             shutdown

linux/crates/app/src/ui/app/workspace_tabs.rs

@@ -1038,11 +1038,20 @@ fn editor_tab_tooltip(query: &str, label: &str) -> Option<String> {
     if q.is_empty() {
         return None;
     }
-    let preview: String = q.chars().take(200).collect();
-    let preview = if q.chars().count() > 200 {
-        format!("{preview}…")
-    } else {
-        preview
-    };
+    // Use char_indices().nth(200) to walk only the first 201 chars
+    // instead of materialising the whole query as a Vec<char> via
+    // chars().count(). For a multi-megabyte SQL dump this avoids an
+    // O(n) scan when only a 200-char preview matters.
+    let mut iter = q.char_indices();
+    let mut last_idx = 0;
+    for _ in 0..200 {
+        match iter.next() {
+            Some((i, c)) => last_idx = i + c.len_utf8(),
+            None => {
+                return if q == label { None } else { Some(q.to_string()) };
+            }
+        }
+    }
+    let preview = format!("{}…", &q[..last_idx]);
     if preview == label { None } else { Some(preview) }
 }

linux/crates/app/src/ui/history_dialog.rs

@@ -271,7 +271,7 @@ impl Component for HistoryDialog {
             .margin_top(12)
             .margin_start(12)
             .margin_end(12)
-            .margin_bottom(4)
+            .margin_bottom(6)
             .visible(false)
             .build();
         pinned_heading.add_css_class("heading");
@@ -291,7 +291,7 @@ impl Component for HistoryDialog {
             .margin_top(12)
             .margin_start(12)
             .margin_end(12)
-            .margin_bottom(4)
+            .margin_bottom(6)
             .visible(false)
             .build();
         list_heading.add_css_class("heading");

linux/crates/app/src/ui/structure_tab.rs

@@ -26,6 +26,7 @@ use std::rc::Rc;
 
 use relm4::adw::prelude::*;
 use relm4::{ComponentParts, ComponentSender, SimpleComponent, adw, gtk};
+use sourceview5::prelude::BufferExt;
 
 use tablepro_core::sql_ddl::{BuildDdlError, DraftColumn};
 use tablepro_core::{ColumnInfo, ForeignKeyInfo, IndexInfo};
@@ -386,8 +387,8 @@ fn build_column_row(
     let row = gtk::Box::builder()
         .orientation(gtk::Orientation::Horizontal)
         .spacing(6)
-        .margin_top(2)
-        .margin_bottom(2)
+        .margin_top(6)
+        .margin_bottom(6)
         .margin_start(12)
         .margin_end(12)
         .build();
@@ -525,8 +526,8 @@ fn build_index_row(index: usize, idx: &IndexInfo, sender: ComponentSender<Struct
     let row = gtk::Box::builder()
         .orientation(gtk::Orientation::Horizontal)
         .spacing(6)
-        .margin_top(2)
-        .margin_bottom(2)
+        .margin_top(6)
+        .margin_bottom(6)
         .margin_start(12)
         .margin_end(12)
         .build();
@@ -593,8 +594,8 @@ fn build_fk_row(
     let row = gtk::Box::builder()
         .orientation(gtk::Orientation::Horizontal)
         .spacing(6)
-        .margin_top(2)
-        .margin_bottom(2)
+        .margin_top(6)
+        .margin_bottom(6)
         .margin_start(12)
         .margin_end(12)
         .build();
@@ -980,6 +981,14 @@ impl SimpleComponent for StructureTab {
         sql_view.set_left_margin(6);
         sql_view.set_right_margin(6);
         sql_view.set_bottom_margin(6);
+        // Match the system light / dark scheme. Mirrors the editor.rs
+        // hook so the SQL preview's syntax colours track the user's
+        // theme choice instead of staying frozen on the boot scheme.
+        apply_sql_scheme(&sql_buffer);
+        let buffer_for_theme = sql_buffer.clone();
+        adw::StyleManager::default().connect_dark_notify(move |_| {
+            apply_sql_scheme(&buffer_for_theme);
+        });
         let sql_scroll = gtk::ScrolledWindow::builder().child(&sql_view).vexpand(true).build();
         let sql_page = view_stack.add_titled_with_icon(
             &sql_scroll,
@@ -1607,6 +1616,20 @@ fn combo_entry(combo: &gtk::ComboBoxText) -> Option<gtk::Entry> {
     combo.child().and_then(|c| c.dynamic_cast::<gtk::Entry>().ok())
 }
 
+/// Pick the sourceview5 style scheme matching the active Adwaita
+/// light / dark mode. Called on init and on `connect_dark_notify`
+/// so the SQL preview tracks system theme switches.
+fn apply_sql_scheme(buffer: &sourceview5::Buffer) {
+    let scheme_name = if adw::StyleManager::default().is_dark() {
+        "Adwaita-dark"
+    } else {
+        "Adwaita"
+    };
+    if let Some(scheme) = sourceview5::StyleSchemeManager::default().scheme(scheme_name) {
+        buffer.set_style_scheme(Some(&scheme));
+    }
+}
+
 fn driver_display_name(driver_id: &str) -> &'static str {
     match driver_id {
         "postgres" => "PostgreSQL",

linux/crates/app/src/ui/welcome_view.rs

@@ -80,6 +80,10 @@ impl SimpleComponent for WelcomeView {
             .hexpand(true)
             .vexpand(true)
             .hscrollbar_policy(gtk::PolicyType::Never)
+            // Cap the scrollable content's natural width at 560px so
+            // narrow viewports don't push past the window edge.
+            .max_content_width(560)
+            .propagate_natural_width(true)
             .build();
         let outer = gtk::Box::builder()
             .orientation(gtk::Orientation::Vertical)
@@ -91,7 +95,6 @@ impl SimpleComponent for WelcomeView {
             .halign(gtk::Align::Center)
             .valign(gtk::Align::Center)
             .build();
-        outer.set_size_request(560, -1);
 
         // Single CTA on the populated page: the "+" button in the
         // group header. Previously we also rendered a bottom pill