perf(send): poll the clipboard for the paste hint only when it helps

The send tab read the OS clipboard every frame the paste button was
hovered, to preview a pasted address. SDL's clipboard read is a
synchronous round-trip that can stall, and the preview can only ever
latch when the recipient field is empty, so most of those reads were
wasted work.

- The clipboard is now read only while the button is hovered and the
  recipient field is empty, polled once on hover entry and then at most
  twice a second.
- Trimming the clipboard text moves to a shared helper that also bounds
  the copy so a pathological multi-megabyte clipboard is ignored.

Tests: the trim helper handles null, surrounding whitespace, an
all-whitespace string, and the length bound.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/send_tab.cpp

@@ -1416,21 +1416,20 @@ void RenderSendTab(App* app)
             paste_hovered = material::IsRectHovered(pasteBtnPos,
                 ImVec2(pasteBtnPos.x + pasteBtnSize.x, pasteBtnPos.y + pasteBtnSize.y));
 
-            // Handle preview state — don't modify s_to_address, just store preview text
-            if (paste_hovered && !s_paste_previewing) {
-                const char* clip = ImGui::GetClipboardText();
-                if (clip && clip[0] != '\0') {
-                    std::string trimmed(clip);
-                    while (!trimmed.empty() && (trimmed.front() == ' ' || trimmed.front() == '\n' || trimmed.front() == '\r' || trimmed.front() == '\t'))
-                        trimmed.erase(trimmed.begin());
-                    while (!trimmed.empty() && (trimmed.back() == ' ' || trimmed.back() == '\n' || trimmed.back() == '\r' || trimmed.back() == '\t'))
-                        trimmed.pop_back();
-                    // Preview any address, a mining one too: once pasted, the field explains why it can't be paid.
-                    bool looksValid = dragonx::util::isDragonXAddress(trimmed);
-                    if (looksValid && s_to_address[0] == '\0') {
-                        s_preview_text = trimmed;
-                        s_paste_previewing = true;
-                    }
+            // Handle preview state — don't modify s_to_address, just store preview text.
+            // The OS clipboard is only worth reading when the result can latch (field empty), polled on hover entry
+            // and then at most every 0.5 s (SDL's clipboard is a synchronous round-trip: 1-5 ms on X11, can stall).
+            static double s_nextClipPoll = 0.0;
+            if (!paste_hovered) s_nextClipPoll = 0.0;             // poll immediately on the next hover entry
+            if (paste_hovered && !s_paste_previewing && s_to_address[0] == '\0' &&
+                ImGui::GetTime() >= s_nextClipPoll) {
+                s_nextClipPoll = ImGui::GetTime() + 0.5;
+                // Copy immediately: the backend frees the previous clipboard buffer on the next call.
+                const std::string trimmed = dragonx::util::trimmedClipboardText(ImGui::GetClipboardText());
+                // Preview any address, a mining one too: once pasted, the field explains why it can't be paid.
+                if (!trimmed.empty() && dragonx::util::isDragonXAddress(trimmed)) {
+                    s_preview_text = trimmed;
+                    s_paste_previewing = true;
                 }
             } else if (!paste_hovered && s_paste_previewing) {
                 s_paste_previewing = false;

src/util/text_format.cpp

@@ -118,5 +118,18 @@ bool containsIgnoreCase(std::string_view haystack, std::string_view needle)
     return false;
 }
 
+std::string trimmedClipboardText(const char* clip, std::size_t maxBytes)
+{
+    if (!clip || clip[0] == '\0') return {};
+    std::size_t n = 0;
+    while (n <= maxBytes && clip[n] != '\0') ++n;
+    if (n > maxBytes) return {};
+    const auto ws = [](char c) { return c == ' ' || c == '\t' || c == '\n' || c == '\r'; };
+    std::size_t b = 0, e = n;
+    while (b < e && ws(clip[b])) ++b;
+    while (e > b && ws(clip[e - 1])) --e;
+    return std::string(clip + b, e - b);
+}
+
 } // namespace util
 } // namespace dragonx

src/util/text_format.h

@@ -82,5 +82,9 @@ std::string truncateMiddleToFit(const std::string& s, int minFront, int minBack,
     return build(lo);
 }
 
+// Clipboard text -> trimmed (ASCII whitespace ' ', \t, \n, \r) candidate string. Empty for a null/empty clip or one
+// longer than maxBytes (an address is < 100 bytes; this bounds the copy of a pathological multi-MB clipboard).
+std::string trimmedClipboardText(const char* clip, std::size_t maxBytes = 1024);
+
 } // namespace util
 } // namespace dragonx